There are lots of things that are traded way when it comes to government.
The libertarian solution to the Teachers Union issue is to have schools run by private organizations not attached to the government, thus would be free to unionize and have all the union benefits.
So long as they are a public employee who vote, and who use their power to have outside influence over the ones who are supposed to be representing all taxpayers not just the ones in the union, there is a clear conflict of interest when it comes to PUBLIC sector unions.
Want to be in a union, support backpack funding of private schools and end public schools
Hell even FDR was opposed to Public Sector Unions