While I don't personally care for South Park, I abhor censorship in any form.

If Trey Parker & Matt Stone told the network: "Look, we're not comfortable airing the episode as is due to the death threats," and said episode was subsequently altered? Cool. If the network told TP & MS: "Hey guys, we can't air this one as is and changes must be made, or we can't air it;" leave the decision up to the creators. But to alter SP's content without their knowledge or consent is wrong and (i.m.h.o.) a violation Free Speech.

*Note: One thing I do like about South Park? It panders to no one; equal opportunity satire.