Of course no one will hit on her while she's wearing hijab. People don't hit on nuns or women wearing conservative Jewish clothes either. It's a universal social cue that says "If you're not into what I'm into I'm not engaging with you, so don't even bother." It's not that the men all of a sudden "respect" her because she's dressed conservatively, in the US they simply figure they won't get anywhere by talking to her. Just ask women in Saudi Arabia, they all wear hijab and are many are constantly getting harassed via Blackberry and text message from unknown males. Same with Egypt, in Cairo women who wear hijab are verbally harassed 90% of the amount that expat women are.
The real issue here is that people (men and women alike) need to be educated on what proper social interactions are. We shouldn't be afraid of talking to each other on the streets, but if a man walks down the street saying "hey mami" or "sweet a**" to EVERY female he passes, there's something wrong with that! Someone has to teach these young men that catcalling in the streets is neither a great way to meet a lady nor a proper way to interact with females.