A couple of thoughts besides many rescue's not being legit is that if a rescue adoptes all dogs many of them would not know about the word teacup but if it is a Yorkie rescue and they are using that word then I would not trust them.
Again the 501K does not mean they are reputable because anyone can write it on their website but this website will help you look a rescue up to see if they are approved...the name can be different so this is not a 100% effective but another clue to look at.
GuideStar nonprofit reports and Forms 990 for donors, grantmakers and businesses
Who are the rescues you are talking about? Do you mind posting them?
It is very difficult for petfinder to check everyone out but I agree with you ..there should be a place to report bad rescues but in this case it still might not be a bad rescue...