Buy brand for 6300 IP then buy the skin for teh RP...
If you don't own a champion, you can't use the skin because you can't _play_ the champion. It would be a huge trap to new players to say 'HEY, here's this awesome skin you can purchase!' 'Oh wait, did you want to use it? *flip the bird*' (Really REALLY bad idea to do this.)
And yes, part of it is marketing; if you want a skin _now_ but don't own the champ, you either need to have the IP (don't waste IP on blind purchases) or fork over some green.