WIY
I believe that the problem IS that you bend over to bowl.
Bottom line is that bowling is an extremely tznius activity. If you dont want to you dont have to bend. I went on a date with a pretty Yeshivish guy and we went bowling (I chose the spot) and he looked away when it was my turn. You know what GOOD FOR HIM! Girls dont need to be locked up because a guy might see something. Whats next, girls cant go outside because they might drop something and have to bend down to pick it up?
I am not a fan of girls saying “well the boys dont have to look” when doing something immodest. In this case the girls are doing nothing wrong, if a boy thinks he will see something wrong or beneath him then HE can take the steps to protect himself.