First of all, this is a cash game. Secondly, why did Player B muck his hand? After calling $25 (because Player A only tossed $25 into the pot), I would agree that both players are only competing for the prior wagers, and the last $25 from A and B. There's no way to ever force Player B to the all-in... at least not in any cash game I've ever played in. Why he would call the $25 and then muck, has me puzzled.
I agree with Brian when he says he agrees with the decision of the floor, but I've never used an all-in button so I'm not sure that would have helped in this situation. Are all-in buttons used in cash games, too? If Player B had not tossed his hand away, I would have considered Player A's unclear all-in, a wager of $25. If Player B had not mucked, I would have burned and turned the river and let the hand play out.
Player B's big mistake was mucking before a decision was made by the floor. The floor might have allowed him to retract his $25...who knows? Why call, and then muck?