Originally Posted by
PonyRoar
After chatting with engineering, using player Arctic Isles Dragons in the Arena is complicated. If the Dragon loses, he'll need to be healed, which means he'd have to be pulled from the Arctic world to the old world. Moving him over and putting him back would be difficult and could result in the Dragon being stuck between worlds, among other things.
Using them as Opponents, they're not coming from player boards, so they're not traveling between worlds.
This is why you can fight them, but can't use them to fight.