It is a hostile act to call people out. It tampers with their privacy and their right to pursuit of happiness in the Constitutional sense. This is why laws are being enacted and enforced.
Except people don't enjoy the same expectation to privacy when in public vs when they're sitting inside their home, on private property. Most web-based forums generally ARE legally considered as being public forums, and some websites have even been ruled as "public accommodations" (like hotels, etc), and hence subject to ADA requirements for non-discrimination, etc. Point being, public accommodations are expected to not restrict free speech (that doesn't include the obvious abuses given, eg shouting "fire" in a crowded theatre, etc).
Other 4th Amendment (free speech) rights are at play, as well: some people wish to lob their volleys at others, and then duck for cover, after their feelings get hurt. However, free speech means not just the freedom to spiel your message, but the freedom of others to disagree (which is also something JWs HATE: they'll go to court to fight for the organization's rights to do the preaching work, but conversely deny their MEMBERS of the right to their free speech).
But the fact remains that many public forums ARE considered private property, and all users agree to the particular rules when signing up (and posting rules are right below, in the posting guidelines). I've moderated forums with thousands of members (a forum of DRUMMERS, who actually were remarkably well-behaved and self-regulating, for the most part), and the fact is you play by the owners' rules at the risk of getting warned and/or booted. Their house, their rules.