They are free to complain too, and in this case Musk can fire them and try to find other talented people to replace them - if that is in the best interests of the shareholders and other stakeholders. There is nothing special about Musk's side of the argument or Musk's leverage; the employees have leverage too.

But really the goal is to make SpaceX as productive as possible, to do something productive for the world, not to serve Musk's whims. If there is a trade-off between the whims and the productivity, it's clear which should be sacrificed.