I am sad to say that the two-tier system is partly due to lingering discrimination in some unions. During WWII, when employers and unions in the war industry were required to admit Blacks and Asians, they responded with "separate but (un)equal" locals, which was ultimately declared illegal.

https://medium.com/engendered/before-brown-there-was-james-f132104f9766

The "two-tier" system, and other separate "classifications" in some union contracts still continues that discrimination. Now, it is used primarily as a way to keep women out of predominately-male trades. Employers are happy to go along, as they get to pay one group of workers much less.

For example, we have a case now in which the Pipe Trades and Boilermakers have set up a separate classification for the primarily female "Safety Attendants" who make sure the primarily-male "craft" workers stay safe. The woman must pay the same dues, but are not allowed to vote, are paid much less, and are barred from advancing through the apprenticeship program.

Some unions are at the forefront of fighting this type of discrimination, while others are still trying to keep women and POC out of their trades. The question they all must face is: Which side are you on?