If Robinhood wanted to manipulate the GME market, they would have banned selling, not buying. Driving GMEs price lower increases Robinhoods collateral requirements and pushes them closer to bankruptcy.
Why assume they'd only want to distort prices up rather than down?
On the contrary, it seems to me that "banning selling, not buying" could be a useful tool for helping out some hypothetical buddies who would rather that a large tranche of 115s & 320s don't end up very inconveniently in the money.
