Why hasn't Sona been banned from ARAMs?

Why not just make every champ available in ARAM, so you don't have ARAM accounts stacking the decks.

sona, soraka, teemo, literally any 2 other champions, gg

Because ARAM isn't URF.

Riot doesn't really care about ARAM, which is why there have been no changes at all to the map or any map specific balance changes, etc since about a month after HA went live. There's plenty of things they could do to make the games more fair and improve the quality of life for players in general but they won't. They don't care. At all.

ARAM is doomed to be an utter joke mode run by smurfs who abuse low champion pools to get cheap wins with broken overpowered champions and ruin the map for everybody who doesn't. We'll never get a blind draft queue or a blacklist or anything to make the map fun for players who don't want to be cheesed by cheaters.