Aerbabe,
I've been running into this for years. As others have said, the majority of instructors have no idea of the rules about this. I've had instructors fill in the line in my logbook themselves, as P1S, for a checkout. I now try and do it correctly, and try to tell them, but if they fill it in, or won't listen, I just let it go. It's so few hours in the long run that I feel it hardly matters. Though as someone will no doubt point it, it's important really. I just can't be bothered to carry on fighting about something so trivial.