In the United States, primary elections are publicly-funded (I had trouble finding a direct citation for this but top Google hits are in general agreement that taxpayers pay most if not all the costs). But not all states allow all registered voters to participate. Some states (Wikipedia counts twelve) hold closed primaries, where only party members may vote in a particular party's primary, thus disenfranchising independent voters, and even voters registered to parties that do not hold their own primary. This answer says it is even a crime for an independent voter to attempt to participate in a closed primary election!
Additionally, according to this poll, in 2015 a record 43% of voters were independent. (I could not find figures for the 2016 election but I imagine it is even higher considering the upward trend.) So the effect is that some states exclude nearly half of voters from their primary elections.
That is not to mention other exclusionary tactics such as lengthy residency and party registration requirements, most infamously New York's onerous six month party registration requirement, which exclude even more people.
I understand that the parties are private organizations which are free to conduct their own elections however they like, but their elections are publicly-funded and state-administered. Taxpayers pay for their elections, and government employees execute them, all for the exclusive benefit of the parties. And yet some states make it a crime for registered voters to vote unless they are already associated with the party - sometimes up to six months in advance of an election. Considering how many people this excludes, doesn't this violate the Equal Protection Clause?
And secondly, I've read the counter-argument that voters are free to register with a party in advance of a primary election (however, sometimes no less than six months in advance!), but doesn't this requirement violate the freedom of association, which should implicitly grant a freedom to not associate?
Are there any other Constitutional problems with closed, taxpayer-funded elections that I've missed?
How can closed, taxpayer-funded primary elections, which effectively funnels millions of taxpayer dollars to the parties who benefit exclusively, possibly be Constitutional?