I think this is just a practical consideration. Execution fees are typically less than a few cents per share and are often fractional. It's impractical to offer up dozens of single cent refunds for current and previous customers.
Also, just adding onto my own post, I worked on a trading desk for a large bank. Shortly before I arrived there was an issue with order marking (the details of which aren't as significant and were caused by a genuine mistake). The SEC made us go through every trade the desk had ever handled and identify which orders were marked incorrectly. It was so tedious and painful that the bank wanted to settle and pay a bigger fine if it meant not having to deal with all that nonsense. The SEC were unyielding. The moral of the story is that there is no negotiating with the SEC once they decide they've had enough of you. I'm not sure to what extent RH have institutional customers but institutions have a fiduciary duty to seek best execution. RH are on a real legal ledge here. I wouldn't be surprised if this is just the start of a very challenging era for Robinhood.
