Either everything is OK to poke fun at, or nothing is.
Don't get me wrong, I don't think Boyle is a genius, but I don't see much wrong with what he's saying.
South Park had Cartman impersonating a disabled kid to take part in the special olympics and win money.
Family Guy had Chris go out with someone with Downs Syndrome and actual performed a musical number about it.
Or there's the singing diagnosis of "full blown AIDS" etc.
[video=youtube;2AUd36xcz24]http://www.youtube.com/watch?v=2AUd36xcz24[/video]
Freedom of speech is freedom of speech. The difference is when you're enciting hatred, and Boyle is not doing that - if anything, he's only inciting hatred against himself.