I gotta say- that's pretty clever. Also, I think the biggest reason girls like it is because it's a throwback to when we were kids, and we could pee in our pants all we wanted. Also, when you think about it, our first ever experience with peeing in our pants after diapers was a positive one- it probably felt good. We just don't do it because the behavior underwent extinction- that is, we were forced to not pee only in toilets and like it.
What does that mean? I bet that if you could get a girl to pee her panties like 5 times in a row, she would either become addicted to it or have to potty train all over again- great way to get some real life desperation stories!