How can CFL teams own rights on NFL players?

[Shipyard]

I believe players on the neg list are mostly players who have a slim chance of signing up here anyways. Last year we had Cam Newton, HAM had Tebow, MTL had Russell Wilson and I believe BC had Kaepernick. These are the players I believe that the club would sign immediately without having a tryout. The guys who participate in these camps are just free agents maybe a few were invited from the neg list but I don't think it's likely

note that all of those guys were put on lists years ago when they were freshmen/sophomores in college and no one except the scouts knew who they were