In some cases, permabans are not necessarily permanent. For example, Riot will "permaban" an account that is compromised, but the original owner can recover it. In rare cases a ban was simply a mistake. If they delete accounts they will inevitably run across instances where they need to unban an account, but now it isn't there and a player has been horribly wronged. It's just better to keep it. In the past there was a program for getting your ban lifted by playing a new account to level 20 and proving you had reformed; the reform rate was abysmally low so that program was sacked, but it's a precedent that explains this, and it could come back in the future in one form or another.