If we assume for a moment that Robinhood is acting in good faith (Seems like a stretch but work with me here).
It's possible they are doing this to protect investors. It's pretty clear GME is going to crash at some point in the near future. Having thousands of investors on their platform go broke at once is just bad for everyone (who isn't short). By blocking these trades, RH is protecting investors from potential loss.
In a way, it's almost like preventing someone from investing in a Ponzi scheme. While it might piss you off, it will save you in the long run.
Of course... that's assuming RH is acting in good faith.