That would be right if we were talking about a trading platform, but what we are actually talking about is a brokerage. A brokerage places orders on trading pl

That would be right if we were talking about a trading platform, but what we are actually talking about is a brokerage. A brokerage places orders on trading platforms on behalf of its customers and brokerages can make various rules about the orders they will or will not place. For example, Robinhood has restricted trading of box spreads for quite a while following an incident of attempted fraud on their platform (which was partially their fault to begin with). Brokerages also have leeway to stop potential crimes they observe their customers committing, which is what happened here. For what it's worth, Robinhood was not the only brokerage that is restricting trading GME. TD Ameritrade, Interactive Brokers, and WeBull also have restrictions in place right now.