They could face lawsuits from investors in Melvin and other hedge funds, who will allege that the brokers knew they were facilitating intentional financial harm. It doesn’t matter if these are likely to fail; they are expensive lawsuits to defend against and therefore may result in settlements.
And they could face class action litigation from trial lawyers representing retail investors who lose money when the bubble finally bursts. Again, maybe the brokers will win these suits but they are expensive and bad for the brand. “I used Robinhood and lost my life’s savings” is not the news story they want to see 2 months from now. (Edit to clarify: class action litigators have PR strategies to feed these stories to reporters, hoping bad press will convince their target to settle.)