True, but it can be applied as a tax

Forcing non-members to pay these fees to unions whose views they may oppose violates their rights to free speech and free association under the U.S. Constitution’s First Amendment, the court said in the ruling authored by Justice Samuel Alito.

California has a regulation, passed by law. Thus, like Obamacare fees, it is a tax on labor and a subsidy to unions, by law. Perfectly as legal as obamacare and protected therefore by states rights. 

So, I change my mind a bit.  It is sometimes a violation of a reserved right unless sanctioned by state law.