The exchange hasn't done anything, though.

It's only certain retail brokers, and, mystifyingly, mostly the ones who have hedge funds with significant ownership interests.

Yeah, that’s clearly problematic. I’m no fan of WSB, but it sure as hell looks like Robinhood has done something very bad here.

But the general case of “the market should never interfere with trading” isn’t true, but typically we only allow that at the exchange level.

No, it's not unusual. And, in fact, the exchanges HAVE used brief halts in trading to curb volatility in GameStop stock. But those were done in accordance with standard policies that simply reference a certain amount of change within a certain timeframe.

What Robinhood has done is apparently to ALLOW trading, but only selling, not buying. At a time when a close partner of theirs is at significant risk if the stock price goes up.