If you're into that kind of stuff, you can see how a state mandated primary election can be a violation of a political party's freedom of association. You can't tell a political organization how they're going to decide who their nominee is.
But, I've been wondering if you can tell a political party how to organize itself, from the lowest grassroots level, to how it chooses a chair, and still not violate its freedom of association.
I've been trying out the argument (here and here) that the election of precinct committee officers, mandated by the state, is a violation of the same freedom of association. So far no one has bit, but I think that has as much to do with the sidewayness of my argument as it does that I've come late to both comment threads. Gotta be on me toes.
The reason I think the party's don't mind these requirements because in a way they help build their parties, especially the PCO elections.
Electing PCOs in a public forum (no matter how little participation there is) gives the parties an easy way to keep activists involved in organizations. Because of state law, the people elected as PCOs put together a local organization, its almost like it happens on its own.
Or, it could just be that the state party would want to put together local branches anyway, and that it is codified is just a happy coincidence. And, that local government is on the hook for low participation PCO elections, they don't mind that either.