^^^they don't screen shit.
the problem comes when having to prove it in court. i've seen threads where an artist recreated certain portions of a track that a beatmaker sent that was all samples and never cleared, how could he claim infringement with other peoples work without their permission? and you can't copyright ideas or arrangements.
now if you get something cleared, for samples that end up getting found, you have a new work to register with all parties involved, at least that was what i was told. it's still based on getting permission.
same goes for your beat being turned into a song if placed. a new copyright involving you and the bum singing/rapping/shitting up the track, therefore a new registration that needs to be sent.
@unorthodox, yeah, i was just narrowing down my examples, but it has to still be in a fixed form. if you sing something out loud, never write anything down, record, archive it on tape/hd/etc, etc, then it's just spoken words (maybe it's in the best interest of any artist, even unknowns, who perform live or do a gig/show, to tape it or record it if you're worried about it)
another link for the brits
http://www.copyrightservice.co.uk/