It has more to do with US politics - both the major US political parties lean to the right- and allowing unions encourages socialist ideas that obviously clash with the ideas of the political right. Unions can (and do) become powerful political blocs which means that political parties then have to try to entrench themselves in it to retain some level of influence. But that also means they also have to actually listen to their concerns and politically deliver on some of them, to retain their influence. This can lead to a dilution of the ideology of the politically right, which some politicians and businessmen do consider as a political threat.
