Because she's being presented as a role model. And yes, I feel if a model is unhealthily skinny, she should try to work towards a healthy lifestyle.
As I said, I think it's great she's saying women shouldn't hate on themselves, but I also think she should encourage a healthy lifestyle as well. As it's coming across now, it seems shes telling women it's cool to be overweight and no need to change anything...which isn't a positive message.
Many models are unhealthily skinny, but I don't see anyone expecting them to demonstrate how to be "healthy." We are more apt to hear about some bizarro-land diet.
Ashley, even though it is completely unnecessary, talks about her workout routines, eating habits and all sorts of other aspects of her life.
Just because you are overweight, doesn't mean you should stay home and harp on it all day. Just because you are overweight doesn't mean you have unhealthy habits.
Ashley's message is positive and important. You should accept yourself as you are today. You only get one body and one life - you shouldn't put your life on hold and hate yourself because for some people it isn't "acceptable."
And everyone - no matter what size / weight / shape they are should focus on eating well and staying active. Whether or not that impacts the scale.