They could have... prevented users from exiting their positions? How would that be better?

In the context of what was happening in the market (the "short squeeze" that may or may not have ever happened), allowing sells but not buys clearly biased one position, could be seen as interference.

Blocking selling is a good way to get sued, so brokerages are very hesitant to do it. If the price drops the trader can claim they wanted to sell and are owed the difference. Blocking buying doesn’t have this issue since you can’t sue for theoretical gains.

I think you still can? But yeah probably much harder in court.