I'm using RAFs for over half year and this is how i look at it:
With prepaid codes Blizzard considers code itself as real world good (forbidden to trade for virtual good) and you cannot say you bought gametime unless the person you bought from would log into your account and activate code himself, but then they could screw you on sharing account info.
Technically looking it's not against Blizzard rules as subjects of transaction are in-game gold and in-game gametime and through RAF there is nothing you could call real world good directly in between.
You get rid of both issues with buying RAF gametime.
Even if the guys who sells it cheat they might take back gametime (i'm not sure if they ever did that), but can't punish you simply because you made fair deal not knowing what other guy planned, unless you knew and they can prove it, what is quite not possible.