it's just entertainment, for crying out loud. My generic response is that if we had higher standards collectively, maybe we'd get better entertainment.
My specific response is that Avatar obviously aspires to have a message, whether it be about the eco system, greed, or whatever. So it deserves real criticism.
My third response is that entertainment is almost never just entertainment: it both reinforces, responds to, and creates social and ethical values through the story it presents; see Pierre Bourdieu's book The Field of Cultural Production for more on that topic. To deny that is to allow the thoughts of others as presented in story to replace your own.