I really need an answer to this.

It is ok to go up to someone and tell them their fly is down.

It is ok to tell someone they have lettuce in their teeth.

It is ok to tell someone there is toilet paper stuck to the bottom of their shoe.

It is ok to tell someone that they've dropped something (a doller, a paper...)

It is ok to tell someone there is a hair on their black sweater and then pick it off.

It is ok to tell someone that there is an eyelash on their cheek.

Everytime I see a girl wearing shorts so short that her butt cheeks are hanging out I feel like it would be common courtesy to tell her as in the above situations. As in: you can't possibly know that everyone can see your butt or you wouldn't be out in public? Right? 

Why is this not ok?

I petition to get this added to the list of common stranger courtesies.