The fight in Michigan wasn't over whether someone had to join a union or not. Michigan laws allowed employees to opt out of joining the union. But they were still charged union dues every month and the money went to the union. That's what the fight in Michigan was about. It was about depriving the unions of the money they were getting from non union employees. Ostensibly it was because non union members were getting the benefit of belonging to a union and should be paying dues, or the equivalent of dues.
Non union employees are free to negotiate their own employment terms, free of the unions and the unions don't want some workaholic undermining union employees by working harder and making more money.
Let's try this hypothesis:
The employees of a company were earning $7.50 an hour until they organized and formed (or brought in) a union, which achieved an increase to $15 an hour -- plus a lot of additional benefits like a 40 hour week, overtime pay, paid holidays, paid vacation, coffee breaks, etc. The union contract allowed the company to hire non-union workers
who enjoyed the same benefits the union members had fought and sacrificed to achieve but the non-union workers are required to pay the same dues as active union members.
Based on the pre-union rate, a 40 hour week at $7.50 an hour would gross $300 (with no benefits). Based on the unionized rate a 40 hour week at $15 an hour grosses $600 -- with a lot of valuable benefits. But the union dues are $12 a week, which the non-union workers are complaining about!
Does that clarify the situation a bit? The scabs are paying the union $12 for getting them $300 more than they would be getting were it not for the union, not to mention a host of extremely valuable benefits.
These people walk into a $15 an hour job, or a $30 an hour job, and don't give a moment's thought about how that hourly rate, and all the benefits, came about! They think the employer is simply benevolent.