What are the rules on buying gold on WoW?

I've read the ToS and RLT (If thats what you call it) seems to be against the rules. I have recently read that Diablo 3 has a real money auction house, and I also read later that there was a way to buy gold from Blizzard. I read that you can buy a guardian cub, then sell it for in game gold. So I'm confused; is Blizz against RLT? If so, why?

I can see that if blizz allowed people to buy gold it would make the game unfair, and the richer people would have more gold than everyone else. But I'm wondering why Blizzard would ban RLT but make the guardian cub tradeable.