Well, Freeware is not a new concept

Quite a few passionate programmers roaming out there.
Aside from them, you've got some apps which are a cheaper version of the paid ones, and some apps which just serve to promote other producer's products.
Finally, unfortunately, there might be some scammers in the app store. Unlike in iOS, in Mac up until recently you didn't have sandboxing, that means that a malicious app could've stolen your data, passwords, etc...