Because they think that she'll end up giving them something (you know exactly what I mean) if they're nice to her. People don't give a shit if someone's in a relationship anymore, they'll do whatever it takes to get the girl. Personally, I treat girls and guys the same online and in real life, and I'm not "Mr. Steal Yo Girl" which is what everyone seems to be turning into nowadays. But yeah man, that's really the only reason why they're like that man, they want some you know what.