Federal law 'prohibits' a lot of things. In real life we know they rarely enforce any laws unpopular with whoever appoints the AG and prosecutors in general, like Eric Holder not prosecuting Obama's and Democrats' organized crime activities, working with foreign govts. against American security interests, ignoring blatant hate crimes when they were commuted by blacks.
All they have to do is ask their first group of shop floor employees, usually a handful of company men themselves, to form a union; that way they can avoid problems with later attempts to organize one by a union they don't control. In' right to work' states they can be formed by just 3 employees, like one of the ones I helped vote out. The union was already 'there' before the even began to look for the vast majority of their new employees. It takes a year after voting one out to try and organize another one.
The craft union movement was the first union movement in the U.S.; it's based on the English craft guild model. Like I said, a union is only as good as its membership, and many unions were run by idiots by the 1980's.