They have no such duty though. They have no ground to say what trades should or should not be made. That responsibility lies with the SEC who have circuit breaker rules that apply to everyone.
Locking out only retail investors while institutional investors can still trade can very easily be branded market manipulation under the guise of "protecting their clients". Why are they not "protected" from buying stock to close their short positions? In fact they didn't disallow retail investors to trade, locking them in, they only allowed them to close positions guaranteeing that the stock would go down. I don't think they have a leg to stand on, personally. At best they could put warnings, but not suspend trading.
EDIT: I saw that tweet saying they started closing positions on behalf of their clients. If true, I don't know how that is even legal.