Well I think everyone gets atleast one shirt from disney when they visit, they need some kind of souvenir. That would be why the year shirts sell and why certain groups buy them. It is the same reason why we have attitude shirts in the parks and they make so many of them.
I bet that disney sells alot of the generic shirts that we complain about because if they didn't, they wouldn't be producing them. Disney can sell these shirts because people want something they can wear when they get home to say that they were at disney.
I really don't care about most of the mickey shirts that they sell, they aren't that bad. With mickey shirts, the designers have more latitude with what they can pull off; the designers aren't hemmed in like how it seems that ever tink shirt is something with the person being a tease or a grumpy shirt is that person is grumpy. I know that these characters are typecasted into single qualifiers, but they have acted in different ways.