Most people who don't join a union because of the financial costs which can be substantial. Others don't join because the union is worthless in protecting their interests (like Washoe Education Association). However, not paying the dues makes the matter even worse.
My school district up here has a "fair share" clause in the union contracts. It isn't the same thing as paying the full union dues, however:
The fact is that many contracts, especially in the public sector, respect the rights of the worker and their choice to join, or not join, the union and have a negotiated agreement with the employer for what is know as a “fair share” agreement. To people who many not be covered by a collective bargaining agreement in their workplace, this is a clause that basically means that if you benefit from the contract, ie work her under this agreement, then you must pay a fee to cover the costs of negotiating and administering the contract. That is it.
It's only fair, after all.
For the record, I joined OSEA a couple of months ago. The cost is only about 1.5-percent of annual salary, or about 15 a month.
Employees up here also have the right to strike, unlike Nevada's public employees.