Can condo association be selective with their pet policies?

I am buying a condo in Wells, Maine. They will not approve my 6 lb dog. There are others here, much bigger than mine but they say that they are part time. Mine will be as well since I am not here in the winter. One of the other owners told me hers is here full time in the summer as well and weekends throughout the year. Can they legally pick and choose who they approve or do I have any legal recourse?