Regardless of the exact technicalities of it, the broker has legal obligations in many ways, and it could have also been to keep them out of legal trouble.

Look, the whole situation was extremely visible, and there are politicians who want blood of wall street for their supporters. The general consensus of most critics who knows the mechanics of the market generally are more in line with that there needs to be more regulation to prevent it in the future -- but few if any who believes anything illegal was done.

I'm not exactly sold on unethical or intentionally illegal things done, but I will say that I wouldn't be surprised if it's found that this was a "bug" in the entire system that was warned about for a while that everyone was just too complacent to fix. Basically, it never happened so no one cared to prevent it.

I may be very well wrong and we can find there was deliberate manipulations done with tons of hard proof-- but it still doesn't hurt to make sure you're always trying to learn more about this incredibly complex system.