If you use other people's prefabs you should give credit to that in the Readme.txt file, I think, but people don't have to upload their prefabs if they don't want to, so it's not like they're stealing other mapper's work. When people make mod's, they often don't do it onlone, they work together with other people.
I agree with David, that prefabs have a lot to go with mapping. Trapt, you said this is a mapping and that it should have a prefabs section. Well, how do tutorials, forums, and competitions have anything more to do with mapping than prefabs? They all contribute to the site and make TWHL what it is.