Does a broker have a legal obligation to allow you to purchase any stock? I am genuinely asking as IANAL. I can't go into an Apple Store and demand they sell me a Google Pixel. It seems like these places should have every right to choose what services they offer or products they sell.
I wouldn't do business with Robinhood for a number of reasons that go back before this recent spat of issues, I am just not sure preventing purchasing GME is the worst aspect of this.