The problem is Boeing's management, not the NLRB, not the union, not Washington and not South Carolina.
In directly relating the problem to the union, Boeing stepped over the line and was acting in an anti-union position, which it isn't allowed to do under federal law.
The NLRB is only enforcing the law.


Re: “Boeing's decision to move to South Carolina has turned into a legal nightmare”
TITLE 29 > CHAPTER 7 > SUBCHAPTER II > § 158 (c) [Expression of views without threat of reprisal or force or promise of benefit]
Boeing specifically said they were moving construction outside of Washington because of labor's actions. This was a clear threat to the union: "Because you don't behave the way we want we are punishing you."
The expressed their views and threatened reprisal.
This is the very definition of an unfair labor practice.