Unions are different from state to state. I am a member of the NEA/IEA (National Education Association/Illinois Education Association). The locals have more power then the state in Illinois. We do have a closed shop, however you can be a fair share member if you choose to. That means you do not pay full dues to the NEA/IEA as they take the proportion that goes to political activities out of the dues payment. This is decided by an arbitrator I believe but could be wrong.
We had to negotiate the closed shop before that you did not have to join. We had two teachers that were not members for political reasons but paid the local to negotiate for them. They didn't have to but did it out of respect. It is not only negotiations that go on. We are also the legal representatives when there is an issue with a staff member. This prevents an unethical administrator from disciplining an employee out of spite. It also make sure they dot the "I's" and cross the "T's" for termination.
The other thing we do is file grievances if the contract is not followed. This is usually as a last resort as most issues are settled informally. I am fortunate to be on the negotiation team in our district. We use win win bargaining tactics with the board and administration. Both sides are trained in interest based bargaining and on most issues come to a mutual agreeable solution with out much posturing.
In Wisconsin it is different. The regional had total power over the locals. Instead of the region working for the locals like Illinois in Wisconsin the locals worked for the region. Of course that is now a moot point.
In both states there are laws about maximum raises as well as property tax caps that kept the salaries in check.