One possible legitimate reason is the risk to Robinhood themselves. They allow margin trading and I think anybody would agree that buying GME at these crazy prices on margin is incredibly risky. If the traders can't pay, Robinhood or their bank/insurers are on the hook. Also even though the narrative here is Robinhood traders vs hedge funds, surely there are also a lot of shorts on Robinhood too, so there's more margin risk from those too.

However why not just disable margin trading on these stocks instead of shutting them off altogether? I don't really know enough to say. Maybe there are additional risks somewhere unrelated to margins? Or maybe the upstream market makers are forcing their hand. Or maybe it's some combination of reasons, including pressure from people who are on the losing sides of these bets.

> Why should Robinhood pick a winner (siding against their own customers)

As others have often pointed out, Robinhood is in some ways analagous to social media companies. Their end users are not their customers, because it's a free service. The customers are the businesses on the other end, in Robinhood's case the market makers who are paying for the right to front-run trades. If this is no longer profitable for them due to crazy volatility, they can apparently stop allowing trades at any time.