Well, banks are earning money, and if it ends up this money is essentially just a "gift" to banks from the rest of us in society, that's a problem.
If you object and say "but they deserve these profits!" then you're arguing that they are contributing to productivity (and then the profits would be fine)
The only reason people think that banks profits are undeserved in comparison to the other industries, is because people don't care to understand banking.
If you take, for example, pharmaceuticals or the food industry, you'd find a lot more f'ed up things, with people massively taken for a ride. But apparently it's not hip to hate on those industries yet.
If you object and say "but they deserve these profits!" then you're arguing that they are contributing to productivity (and then the profits would be fine)