Mainly, because if you do that, if you go on the internet and say they're a bunch of idiots, and all, you're no better than they were. Also, we have here what are called "crimes against honor" and both actions would qualify, so I'd much rather have them be MADE to apologize for, well, being idiots (not to mention the shame of having to do so in public, it's well worth it in these cases), and preserving the image of the people they offended, than lowering to be like them and simply "offend them back".