Its a private company providing a service for FREE. YOU ARE THE PRODUCT. Just like twitter, they are allowed to control what happens on their platform, and they make no guarantee about reliability or free trade.
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.