Someone remind me again how this is even legal? You can SELL but not BUY, thus automatically suppressing all stock values?!
They are not "allowed to control what happens on their platform." in the sense of Twitter. They have a fiduciary responsibility to their clients and the reliability of their platform is actually part of their ability to operate as a brokerage.
The service being free or not has NO impact on the rules they operate under as a brokerage.
Given they are currently arguing in court they have no such duty, but only to act in their customers best interest, perhaps this isn't sufficiently clear to them.
Read section 16.