They had no choice. They literally couldn't obtain any more GME stock. You cannot squeeze blood out of a stone. What do you want Robinhood to do?
The GME situation got into a "sold out" situation. Much like how a toy-store runs out of Furbies back in the 90s, Robinhood ran out of GME-stock to sell to its customers.
They would allow "selling", because Robinhood can then obtain that customer's GME stock, and then give it to another customer almost immediately.
------------
All of this margins and stuff is probably just overcomplicating things. You're basically getting mad at toy stores for running out of Furbies, PS5 (or whatever fad-toy is available) during Christmas. Sold-out means sold-out, they can't sell you anymore.
But they were willing to accept sell orders (aka: buy GME stocks from others), because that would replenish their stocks of GME to sell to other customers. If some PS5 scalper came up to (insert store here) saying "I wanna sell my PS5 at market prices", of course the toy-store would buy the PS5 (and immediately sell it to the next customer for a higher price).