Well, that stinks! It doesn't even sound constitutional. But I'm with you, in that I wouldn't vote in the primary at all under that condition.
When I moved from NY to South Carolina, I was surprised to learn that one can vote in either primary, and you do not have to sign any type of pledge. I actually voted in a Republican primary (and it felt very strange), because there was no Democrat running for the office, and I wanted to have a say in who was the Republican candidate, since that candidate would automatically be the winner.