but, to play devil's advocate, the concept is that the party is a semi-private organization. If you're not a member why would you get a say in what leader the party is putting forth as their candidate (ie: go make up your own party and choose your own candidate).

The flip side is that, in American politics, there's pretty much 2 parties and that's it (most attempts at a 3rd party fail). So, if you're not a member, you don't get a say (vote) in the primaries.

Shrug. It's not a huge issue for anyone, as it's not THAT hard to cross party lines (I'm a Republican...no wait, I'm a Democrat...no wait!)