Seems more like they're trying to get abandoned accounts down to $0 in their wallets so they can cancel all the abandoned accounts wasting space on their server than actually trying to charge people a fee for something.
This isn't about "wasting space on their server", it's about confiscating money left in those accounts. It's illegal for them to just take it, because it's not theirs.
Not to mention that almost every bank has fees too. Overdraft fees are charging you because you didn’t have money!