While I understand all the reasons given, I also believe we can't protect our children 100% against life. Hopefully, we will give them the life skills that they use their brains and can figure out what to do. I would think that a 9yo would be smart enough to say, 'something isn't right. I'm leaving.' If this child can't manage to go to the bathroom alone, how does he go to school ? Manage homework ? He must have SOME critical thinking skills ? If something seems 'off', you can always walk down the concourse and find another bathroom.
ITA with this... at that age, a child should have been taught both in school and by their parents how to handle a situation where an adult is being inappropriate. In the rare event that a preditor does approach this child, and the child just runs away or screams or whatever, the child will not be scarred for life. It would be a terrible thing, but the child would acutally gain a sence of empowerment and pride that they were able to handle themselves. I know- I was approached by a pedophile when I was a little girl. I knew enough to run away and go get my parents- and I was not scarred for life. I felt proud that I had done the right thing, and it helped me build confidence that I could handle myself. When this happened I was in second or third grade, so I was around the age of the OP's son (I think?).
Just my two cents....
eta: Don't get me wrong, I am not saying you should ever purposely expose your child to a setting that you think is dangerous! Or put your child in harm's way, just to build their confidence! I just personally don't think a crouded men's bathroom is a dangerous place for a 9 year old boy, and I think that parents today tend to be very overprotective and don't trust their children's common sense and street smarts as much as they should.