To be clear, RH (and others) blocked their users from trading those stocks. That’s separate from trading halts, which are initiated by the listing exchange based on algorithmic rules and apply for all trades in the US. The broker is in its right to give you a margin call anytime. It cannot liquidate your position if the stock is halted by the exchange, because all trades are halted during that time.

All that said, my personal opinion is that they couldn’t check in a code change to handle the increased collateral requirements, so the only feasible and somewhat legal way to save RH was to prevent its users from buying more. Pretty shitty, but then again, ever since RH crashed on 2020-02-29 because none of their devs thought of leap years, I can’t say I expect very much from them technology-wise.