I get that South Park lampoons everyone, so why is this group protected? Like it or not, when you're dealing with mentally unstable people, sometimes the least desirable way to go is the one that is the safest for all involved. It sucks, but that's how it is. Fanning the flames only makes unstable people even more unstable.
It's like in elementary school. Whenever someone gets called a name and they protest it vehemently, then everybody latches on and relentlessly calls them this new undesirable name. Is it okay defend their asshattery, when the victim is obviously clearly and rightfully offended?
I agree that on the other side, there should be SOME sort of concession, but if it were your religion that were being lampooned, you wouldn't expect to have to make concessions about what you think on it. But I bet you probably would be willing to let it slide. Why? You're a rational huuman being, who can be reasoned with. These people aren't, in my opinion. Doesn't mean we have to cater to them, but it also doesn't mean we have to go pushing their buttons on purpose either.
I have no idea what the appropriate protest would be, but in my mind, this certainly isn't it.