On the one hand, I agree, that has to be frustrating, but on the other hand, they are only human; you cannot expect them to be all smiles and friendly 24/7, especially right before or after a 3-hour show (not sure if they are before or after the show this time around). I know, I know, they should put on happy faces considering the fans are paying extra for this, but like I said, they are only human.
Also, remember that it is called meet and greet. Saying hi or hello is a greeting; having a conversation is more than that, and if they have a conversation with every single fan who goes through the meet and greet line, it would take forever. They almost have to keep their answers short and sweet.