If they'd beaten the man up whilst he was still threatening them they probably wouldn't have been prosecuted. It's because they were running away when beaten - it amounts to assault instead of self-defence (because they were no longer under threat).
It's like taking a hunting rifle with a decent scope and taking a guy who mugged you out from a block away after the fact. It may make you feel better, and many wouldn't see a huge amount wrong with it, but it's no longer self-defence.
Probably would have helped if the uncle/son called the police instead of a spot of vigilantism. Or if the lad had gone to a neighbour for help instead of a "nearby uncle" (the uncle may well have been a neighbour, but the wording makes me think differently).
Am very surprised the breaking-and-enterer got away without prison time though, but I guess a non-custodial sentence is still a punishment. Oh wait, I'm talking about my country, forget that...
+EDIT+
Holy crap ninjas! There were 7 posts when I was typing this.
Ah the rise of internet toughguy-ism. I was wondering how long that would take.
As for "I'd kill them" or "I'd shoot them with hollow points!" or "Kill him, cut yourself*" - this is the UK where such behaviour is frowned upon. I know some states have a "Castle Doctrine" or similar to that effect, but there is nothing in the UK.
You can use "reasonable force" - if your life is under threat then you can (technically, it'll be a ***** to prove though) kill. But once the other fella is running away, and you GBH/kill them then it's no longer self-defence.
And to be honest, if you're willing to kill some bloke who stole a TV (threatening your family or not) who legs it when they realise they're outgunned/outnumbered then you are scum yourself. The fact the fella flees is indicative that they are unwilling to have physically harmed your family, and ergo you are gunning them down in cold blood. If they were in the process of actually harming a family member then go right ahead - I wouldn't have a complaint with that.
*This wouldn't work - an autopsy would be mandatory and the evidence wouldn't stack up - the victim/burglar wouldn't have any defensive wounds, nor abrasion marks on the fists, nor would there be the correct disposition of epithelials for an assault. And your defensive wounds would look self-inflicted.