Right, but as far as their assumption in interpreting the bartender’s question, they would certainly assume he would not ask them a question that they could answer only in the negative or with inherent uncertainly, thus eliminating the possibility of answering in the affirmative. And since the likelihood of each indeed wanting a drink was relatively high (being that they walked into a bar) he would be depriving himself of the profit. (Also assuming he had a personal interest in the profit (either as the owner or commission)
DISCLAIMER: I understood what I said before I wrote it but I make no guarantees as to whether or not it makes sense after I wrote it.