Put yourself in a 15 year old's position - you have no credit card so can't pay for RP with Paypal or Moneybookers, however you do have some cash and could buy some Bitcoin. Send to Riot, get the RP, everyone's happy.
I don't know about you - but where would a 15 yr old kid go buy some BTC with cash, kids at that age can't even drive so I'm pretty sure their parents would be more willing to either pay for it with their CC or just buy them an RP card. If anything it'd be easier for someone to buy a card than buy BTC and just use that. Anyways, not to mention - much of Riot's userbase doesn't know/use Bitcoin so it wouldn't really be adding anything for them.