In no state are any employees compelled to formally join a union. In an American-style “closed shops” in a non-right-to-work state, if you don’t join the union the bargaining agreement can charge nonmembers an agency fee representing most of the dues they would pay as members. Not to mention pressure tactics to join.
Also especially in the trades in major markets unions control apprenticeship programs and hiring halls and make it virtually impossible for nonmembers to get union work, as well as gatekeeping access to apprenticeship programs in the first place.
"For the conversion of his money into capital, therefore, the owner of money must meet in the market with the free labourer, free in the double sense, that as a free man he can dispose of his labour-power as his own commodity, and that on the other hand he has no other commodity for sale, is short of everything necessary for the realisation of his labour-power. "