http://www.seiu521.org/Union_dues_structure_changes_in_2011.asp
"Even though times are tough and we're all having to tighten our belts," said RoseAnn Dominguez, SEIU Local 521 Treasurer, "please remember that our dues are what helps our Union keep going in the fight to help us win decent contracts, minimize or stop takeaways, and put worker-friendly lawmakers in office to fight for our rights." "
Where in the above statement does it say anything about voluntary contributions !
What I told you is exactly what happens, I guess you want to believe something else more sinister?! Look at DOL statistics on whether a Union employee has better compensation and benefits or a non-Union employee. Not rocket science.