What would be the point to make something illegal, because someone appears to have a eating disorder?
To make promoting it discouraged? Yes.
To make it seem criminal because someone has a disorder? Absolutely not.
All problems are always technical, they can be solved by understanding the issue, finding the underlying causes and then reason with that problem, then you can implement a solution.
The brain will base it's "free will" from experience and culture (including social pressure and environment), your mind is always indoctrinated based on what you are exposed to.
The problem with today's society is that they portray what is good or bad through the use of the media (mostly);
Obesity seen as something that everyone should avoid, see it as a disease or an illness.
Then you portray what is "Sexy", most of which when they portray what is sexy is a woman who has no fat on her.
Finally, you get people told what is bad and good for you, such as eating.
This is where the eating disorder comes into play, if someone happens to be exposed to living a life of looking "sexy", or not look obese, then they will take these measures to accomplish what they think they've done.
We need to have a society that has people who are educated in understanding ourselves, and we need to have a society that avoids defining things such as sexiness and ugliness. That's how you properly educate people which also increases individuality as well.
But the sad thing is, though, we assume that someone suffers a problem just because they felt like to, when in actual fact, we lack in finding the underlying reasons why it happens in the first place.