Preventing their users from selling out of a position would be much, much more serious than preventing them from adding to a position. Incomparably so.

Have you not read about the fire they were under a while ago for making it too easy for people to lose money? Making it so people would be stuck in their positions would be insane, way more than their current actions.

They stopped both buying and selling, except to close existing positions.

The stock dropped right after they banned buying. It would have kept going up otherwise.