They're allowing the stock to sell, they only disabled purchases. This will clearly make the stock go back down, benefiting Robinhood's investors over their users.
First, Robinhood does not make the market or fulfill the orders, so if their market-maker has no one selling a stock they have little option other than stopping people from making purchase orders. You need both sides for a liquid market.
Second, IF these are margin positions then it is not only allowed and legal but also typical for the broker lending on margin to close out the position unilaterally. IF these are cash positions it is a completely different story.
No warning from Robinhood on the current issue either or I would have pulled my sell order.