Surely they aren't restricting outright buying. Assume this is a limitation on use of margin for investing in these stocks. If so, I'd say it was a good move, since if the margin is collateralized by the underlying stock, and the underlying stock has a fundamental value of close to zero, I probably wouldn't want to be lending money secured by these stocks either.
No, fully preventing all users from buying the stock at all. Doing it for AMC, BB, and all the 'meme' stocks.