Their legitimacy might be questioned based on their compliance, it seems that they're not compliant at all and this is their business. If OFAC has already given the sanction for their currency to be traded but BingX isn't compliant at all, that's their problem and they made themselves aware of future sanctions that will hit the exchange itself. But I think that this won't be a problem anymore globally, many exchanged have stopped their support here and there and even had hard time complying in US' standards. And as for the basis of these sanctions from US, they might or might not be obliged to follow and will just leave the region that they're supporting covered by the US laws. Anyway, if Bingx doesn't care at all, why should we be?
Maybe BingX doesn't care at this point, or at least they seem like they don't care about complying with anyone's regulations, but at some point when they do care, all the damage will be on their users.
I am not their user, but I believe that all their users should be concerned.
I'm not a user either and you're right that even if they don't care about such compliance and sanctions for the consequence of this decision that they have. Their users need to think about this action that they're having right now, we're not giving them a scare but they have to think as many times as they can if they've got funds still there.