First a word about the reasons why union membership is a human rights issue. Article 23 if the Universal Declaration states:
Everyone has the right to form and to join trade unions for the protection of his interests.
However, union membership is not only a right in itself, but also a prerequisite for other rights, such as good labor conditions, equal pay (see also art. 23), the right to strike (art. 8 of the International Covenant) etc. The right to union membership is also a special case of the freedom of association (art. 20 of the Universal Declaration). Obviously, none of this implies that labor unions are inherently beneficial, that the right to strike is absolute and so on.
It’s well-known that union membership was never very widespread in the U.S. and has been declining steadily:
(source, I must say I’m a bit surprised to see France here at the bottom)
Here’s a map:
Apart from the right to be a member of a union, there’s also the separate right to engage in collective bargaining (negotiations between employers and the representatives of a unit of employees aimed at reaching agreements which regulate working conditions). However, it’s a lot more difficult to engage in collective bargaining if unions are practically non-existent. Here’s a map about collective bargaining rights in the U.S. public sector:
More human rights maps here.