Samsung has a lot to lose by alienating Apple. Apple is a huge customer, very few organizations buy in the volumes that Apple works with. Losing a big account would suck, but Apple pouring billions of dollars into Samsung's competitors to improve their manufacturing technology will make a large impact on Samsung's ability to compete.
Also consider that Apple's order volume enables Samsung to run their factories at higher capacity, reducing overall operating costs by reducing or eliminating downtime. In some cases this could mean that a factory would not be able to operate profitably. Could Samsung's US fab maintain their price points and sustain their infrastructure development plans without Apple as a customer? I have no idea but it doesn't seem like an obvious answer.