I would argue that it should be! Once a person knowingly posts something on the internet it's in the public domain, denying others access to it for anyreason is wrong.
Illegal, unethical or preventable? No. Although you could set up your forums so that posts can't be edited, it's a bit restrictive.
What we think of as obnoxious and inconsiderate to the greater good might seem like common sense to those more fearful of displaying their thought to an unknown and uncontrolled audience.
BUT, I'll bet a LOT of this behavior (esp. around here) is due to students attempting to hide their trail of virtual cheating from their teachers prying eyes. 8^O
In this case, though, I'm betting it's just a simple case of csmissunderstanding.
If twicker kept enough space for the history of new users' posts and there was a feature that allowed us to revert a post by a certain number of edits, this would be a non-issue. :/