No system is perfect- but usually the offenders will want to vote in the Democratic primaries too. It is a bit inconvenient to switch parties constantly if you have to keep changing your registration, so it slows the left-turn effect down.
What does it matter when Obama ran unopposed.
I don’t think you see what has been happening here. Or perhaps I’m missing something.
Dems vote heavily for non-conservative Republicans (RINOs) in the Repub open primary. Then there are two non-conservative candidates on the ballot for the same office in the General Election, in which any voter may vote for any candidate. Thus, for example, we consistently receive both Lindsey Graham and a Democrat on the ballot in the GE against the wishes of all conservative Republicans. And there are plenty of us. That’s why the jury is rigged.