Who do you think those shares are being sold to? Robinhood's brokerage side is restricting trades, but their exchange (the "shadow" exchange they use to make money without charging commissions) is still allowing buy orders, or else they have effectively suspended GME on their exchange and have to route the sell orders elsewhere. Again, it is not possible for an exchange to only allow selling of a stock, because every share sold is a share someone else is buying.
Plenty of American brokerages have not imposed any restrictions either, and again, a brokerage is not an exchange and it is important to know the difference if you are going to be a self-directed investor.
