I don't see any moral justification for violating someone's contact liberty. If someone does not want to employ unionized workers, or workers who go on strike, they should not be forced to. Freedom is a two-way road. You have a right to do whatever you want including organize and strike but you do not have a right to force other people employ while you do these things.

I simply cannot wrap my mind around the idea that it is morally permissible to restrict freedom-of-association/contract-liberty in this way.