Because it's a very non-confrontational way of expressing your view and "voting" with your time & effort. Why fight an entity that you give value to anyways through your employment, it almost cancels out. Rather work to better a company that aligns with your own views while at the same time potentially calling out the other company.

Then again, if they choose to stay and their contract says they aren't allowed to do this (assuming its reasonable), then they should also accept it when they get fired and move along.

The issue in this case was specifically that the employees did not pursue internal avenues and instead went public with it. Nothing stops you from writing an email to the CEO and express your concerns. At your own risk, of course.