The audience might not have liked the first movie, but I think the actors really enjoyed making it. They are all friends who want to let off some steam now and then. They did that with Expendables 1 and somehow it also brought them money.
In other words: they were throwing a party for themselves last time and the party gave them loads of money instead of costing them. This time they are building a new party and they invited more friends.
Let them have their fun, there is enough complaining on the escapist already. If you don't like the premise, don't watch the movie and steer some attention that IS worth watching, like the new non-Bayformers Transformers game.