The U.S., as a system of government, does not actually have a primary system. See if you can find it in the U.S. Constitution--I'll wait while you look.
> In our recent election we had two choices for PM from the major parties, chosen by the parties themselves.
This is what the U.S. has as well. The Democratic "primary" is a private process that is not required by the Constitution or federal law. It is set up and run by private citizens for the benefit of private citizens. It is how the Democratic party chooses its candidate, and it works however the Democratic party says it should work.
Participating in a primary is not like participating in a general election. There is no federal right to be considered for the Democratic candidate for president. There is no federal law that says the Democratic National Committee staff has to provide equitable treatment to any particular candidate or campaign. There is not even a federal requirement that a citizen be permitted to cast a vote at all in a primary.
I am hopeful that one of the results of all this hysteria and lawsuits right now is that the courts will help make that clear to people.