First of all, the exchanges could never impose that rule, because in order to sell you need to have someone buying. It is discount brokerages imposing the rule, and they are doing so because they saw what looked like market manipulation and are not interested in helping with that sort of thing. They could have stopped trading entirely, but that would have been worse for their customers, who would have watched their positions lose value and been helpless to do anything about it.
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.
