They are under no obligation to allow you to trade any stock or any form they want. Brokers just serve as a conduit to the market in the form of their choosing. They are a middleman who collects fees in exchange for some risk.
The brokers are bound by a number of regulatory bodies.
The challenge is that these "Regulatory bodies" haven't put in regulations regarding what's happening right now. Let's not pretend all these brokers are doing this because of "risk". They are all doing this to keep their real clients happy. (The Hedge Funds)
The fact that someone could even argue that what's going on here is "Ok, and perfectly legal" blows my mind. This is CLEAR market manipulation. Either enforce the rules for ALL investors or none.