It also keeps other players from trying to influence decisions with chat. Certainly in a live tournament, you'd never see a player go all in and then have the rest of the table shouting, "Call that bluff!!" or "I mucked an Ace!" but the anonymity of online poker is sometimes too much for a fool to bear. It has less to do with you trying to elicit a response from the all-in player, and more with you not having to deal with chatter from everyone else.