I agree that it isnt that simple, but you could still ban it.

Real world policy is rarely black and white, all or nothing. An inability to prevent every instance of something is not a reason to forgo any action

Just because country X has slavery, doesnt mean the US can't ban it. There may be some consequences, and disadvantages to doing so, but that is a trade off, not a show stopper.

Same thing with open source. There is lots of illegal and bootlegged content that is shared on the internet, and cant be eradicated. That doesnt mean it can all be freely bought at Walmart.

It doesn't matter, those states would not be better off if they did not ban it.

They would be better off if you did it the other way around - ban large public models.