Let's take a look at this. Words, especially in the American English language, have a way of constantly evolving and utterly changing all the time. Words like "retarded", "gay", "******", and "[racial slur of your choice]" (my apologies to any homosexual reading this) tend to follow a trend of transcending a specific derogatory specification and turning into a generalized expression of distaste or disapproval. Depending on your location the usage and severity of these words will vary wildly. For example, I live in the deep south where some of the older members of the community will use the term "n***ered" (my deepest apologies to any readers of African descent) to refer to a situation that has been screwed up. They do not mean a person of color, but rather are falling back on the cultural lexicon that they grew up with. This is not trying to justify their usage, but rather to simply explain it.
In short, you have every right to be personally offended by the usage of "retarded", but you should also take into account that the people using it, especially since the Internet is an international medium, may not mean to offend you.