Graduate student unionization has had a troubled past (in the US, the article's author doesn't reference her institution). From my research and what I know at institutions I have worked in, organization has been stymied mostly by a ruling of the National Labor Board that disallows any person receiving "training" who is labeled as a "student" from organizing, hinging on the fact that they are not working a wage job but instead a "fellowship." This distinction can be groan-inducing for academics, but alas.
I don't think this necessarily reflects the "overall defeat of unions in the US", but your philosophical argument does give some food for thought.
For more information, Wikipedia has a summary history of graduate student organization: https://en.wikipedia.org/wiki/Graduate_student_unionization