I think their solution would be to reduce the number of cars made in foreign plants and move that work to the US.

It's pretty simple: the UAW wants to keep their members employed. This isn't necessarily unreasonable. However it is one of the factors which drove the old GM into bankruptcy. Toward the end they were building cars that no one wanted and selling them at a loss because due to restrictive union contacts that was actually cheaper than laying off workers.

Because they don't care if the cars get sold, just if their union members get paid.

I believe it's more about shifting plants to other locations/countries. Existing factories can be re-fitted to produce a different model.