Now this is the big one, a stipulation put in place by the Thalmor that let people crucify them, and yet it's the one that they are most likely in the right to call for beyond a reasonable doubt. It's not a simple measure of infringing on some supposed right to religion, it's about dismantling a bloodthirsty racist cult. They said they would do it, this is the first step of 'purification for all' that was put forward by the Thalmor Creed.
Talos, a righteous god, a grand and wonderful ascended Divine that opened up a glorious age of prosperity and peace... not from the Altmeri point of view. We know that Talos wasn't born a god, how did he become a god? Through being a bloodthirsty warlord? (Redguard) Through unleashing a weapon of mass destruction on a peaceful foreign population center? (On Morrowind) By turning the weapon on his political opponents at home? (Arcturian Heresy) Through viciously depopulating subjugated areas and causing wanton destruction? (Pocket Guide to the Empire 3rd edition) Through having his ethnocentric ideals around human supremacy used to abort a half-breed child from his lover so his legitimate heir wouldn't have any competition for the throne? (The Real Barenziah) Why was Tiber Septim's heritage, loyalty, and motivations questioned even after he had won one greatest and most decisive battle of his career at the battle of Hunding Bay? (Pocket Guide to the Empire 1st edition) Simply put, through what method, and for what reason did Tiber Septim become a god? There is one though, it was spelled out quite clearly in Cyrodiilic culture.
Cyrodiilic and Nord cultures venerate heroes, that much is true, the Elves do it as well. However, while the Elven heroes are often from the first generations of their king, or through certain specific circumstances in the case of the tribunal, the Cyrodiils and Nords will simply set up a shrine and start praising their savior as a God (Reflections of Cult Worship). It's common, Tiber Septim did this to his own predecessor Emperor Cuhlecain by establishing the Cult of Zero-Emperor (Arcturian Heresy, Pocket Guide to the Empire 3rd edition). One of the most peculiar points of note is that the central Cyrodiilic people were not generally in favor of Talos as a Divine, his converts were from three groups of people: the military, the provincials, and those recently assimilated into Imperial culture (Reflections on Cult Worship) so while the altruism behind some of the dialogue of 'The Talos Mistake' might not be correct, the general principle that he was merely a great man and Emperor, is still viewed favorably.
Often the question associated with the Thalmor/Human dichotomay is that it's phrased in the form of a Nord question. ?Is Talos a god?? There's evidence back and forth for this. The Nords have phrased the debate on their own terms by making it an issue of religious freedoms. It bypasses the quintessential Altmeri counter-question which is never asked directly, ?Why are you following him?? (As an aside, I feel into this trap many many times. It's a question always put forward with the unquestionable and unspoken answer that Talos should be worshiped and it's merely a matter of if they can worship him) What is the purpose of worshiping someone like Talos? As a point of note, Reman Cyrodiil had a cult and was worshiped as a hero god, Pelinal Whitestrake did too, so why did Talos become so popular for them?
The reason for this has almost nothing to do with Talos, it has to do with someone far earlier, Lorkhan. Nords in particular have associated Talos worship with worship of Shor, the Nord name for Lorkhan. This has allowed them a chance to transpose Lorkhan worship onto Talos as a Divine all his own. The Nords in particular have ascribed Talos the position of Shor that was never mentioned in traditional worship ?Talos has become Lord of the Divines? (Priest Heimskr, Nords Arise!). It makes sense, Shor and Septim have many similarities: being a warlord with an ethnocentric point of view (Redguard, Pocket Guide to the Empire 1st edition), they were notable for uniting human lands, they were both notable for being vicious, both were ascribed to being pure Atmoran Nords making them the quintessential role models for Nords. But when the Cyrodiilic 'apologist' religion of the 8 Divines were being created, the Alessians understood the value of not including Shor in their worship as it was offensive and vicious and couldn't win converts on its own message of bloodshed and violence (Shezzar and the Divines). They half-heartedly included him as a 'god' but not a recognized Divine in Shezzar. The Nords were unhappy with this but accepted it. When Talos worship became acceptable they simply used him as a method of transference between Shor to Talos and thus had their Divines not only included, but placed at the top of the pantheon.
Ergo worship to Talos is worship to the ideologies of Shor, and thus Lorkhan which is why the Altmer are so displeased. Likewise, this same religion that had already compromised their own gods was forced on them in the form of the 8 Divines. Thus when Talos became the 9th Divine they were forced to worship him too. There are still very very likely Altmer alive that knew Tiber Septim first hand, it's only been about 700 years. Thus they still remember the time when Talos was their cruel, vicious conqueror and humans see it as something that always was and always had been. The Altmer wish to correct this, after all, they have no problem with the 8 Divines, in fact they support this, but the worship of Talos is simply worshiping the memory of a person who's quintessential personality was to demean the accomplishments of non humans and subjugate them under his rule.
There is one group that would whole heartedly agree that Talos was a god, the Psijics. However, that doesn't make him a Divine, and it doesn't make him good, it makes him merely important. Simply following the path of religious connections in the Altmeri tradition, this would make Talos a paradox, a Sithian creation that was by their very definition, a Daedra, which gives him power, authority, and ruthlessness, everything that traditional Altmeri religion says they are to avoid or else they'll end up like Trinimac. The worship of Talos, is therefor, a corruption in and of itself no matter how an Altmer will look at it. That is the reason why Talos worship is banned.