Belonging to a Union gives you the right to have a say in what your wages, benefits and working conditions are. This is a myth, Unions rarely have to strike in order to obtain decent wages and working conditions for their members.
Union dues are like an insurance premium.
If you have car insurance, you pay a premium. There are no free riders in a Union. Everyone has equal rights and equal responsibilities. The Union is required by law to protect the rights of non-supporters without discrimination.