Because, free software isn't a reference to price and said freedom loving developer would like to get paid while still sharing his code with the masses. This is a great thing for free software. All of the code can be completely free and the developers can still get compensated for their time. Its identical to the way sourceforge hosts open source projects but is add supported. This way, its the developer getting compensated directly instead of having to rely on a 3rd party charity to host their code.
Sure, enterprising users could patch out the adds, just as enterprising users can find open source software at places other than sourceforge, but most won't bother and thats a good thing.