How come girls give away their picture for free advertising use?

This girl I know is absolutely ecstatic after she posted her picture on a makeup website and some advertisers picked it up and put it on a billboard. All of her friends going gaga over how cool she is for being a model. UM, so the company is getting FREE advertising. Why doesn’t she care that she’s not getting any money from this?
Ugh. It’s like she doesn’t need money for compensation because what she wants most is to stroke her ego.