Firstly, this fine was issued by FINRA, not the SEC. Secondly, which laws did they break exactly?

This is a substantial fine for the scope of violations that were being investigated, as this has nothing to do with any of the gamestop restrictions earlier this year but their messaging, outages and vetting of customer expertise during 2018-2020.

It boils down to a $70M fine for not having a high avilability system, being bad at explaining trading concepts and allowing people to take risks that other banks would not. That's quite a reasonable amount.