I'm going to tell my girlfriend that dealing with guys hitting on her is entirely up to her
What motivated you to be so cold?
I want her to stand up for herself
You should step in if people will not leave her alone. If she does not want the attention you should be kind enough to help her instead of leaving her to be a victim.
That's not my problem
It sounds like your mind is made up, and that you want your girlfriend to be fully independent.  However, I have to warn you that your girlfriend likely will not share that view, and that she will probably want and expect you to help her in these situations.  She might decide to end your relationship as a result of your indifference.