FFA is for those that can't afford a subscription or aren't sure they want one, or what have you. If you increase the number of players allowed, why would they ever subscribe? 8 is enough to give you some fun, but not so much that folks don't look at the MA and think "Hm.. should I...?"
So it's not really in there best interests to give people free arenas, because those people then don't need to pay HTC.