Whenever I read about some country creating a law like how France is trying to outlaw inciting thinness, I first think, "Hooray!" What a great idea... Hopefully it will help save the health of some people.
But then I think about how truly chaotic and irrational legal systems really are: why don't they (or why haven't they already) outlaw the promotion of unhealthy behaviour or possibly bad behaviour. That would cover something like this plus all sorts of other things as well like smoking and lack of exercise, and drugs, and so on. Of course, now you're thinking, "You naive little idealist... how could a government or legal system define what is generally considered unhealth behaviour, let alone BAD behaviour." But isn't that what they're already doing as a matter of course? this ban on inciting thinness would have to include some connection to what is defined as unhealthily thin. Criminal law is all about defining what is considered bad behaviour.
Instead of trying to create a list of what is unhealthy or bad, whichever legal effort you're arguing for here, wouldn't it be more practical and helpful to define more generally what we mean by these ideas. By producing books and books of legal definitions and procedures and consequences, all we end up with is a profession of interpreters of those texts, and more and more citizens who don't really know what they can and can't do, or should and shouldn't do. On the unhealthy side of the issue, it's already bad enough given that health sciences are already complicated enough without wrapping the concepts in legalese.
And it's not like we don't have anybody to try to generally but usefully define unhealthy and/or bad... Doctors and other health care professionals are always perfecting and tweaking what they think should be considered unhealthy. As for "badness", beyond the applied definers like police officers, lawyers, judges, and the clergy, there are also philosophers of ethics that think of nothing else all their lives. Sure they all disagree, but so do law makers and medical researchers. The latter make much more headway but of course they have much more funding than "professional" philosophers. Imagine what we could nail down if there were whole institutions of philosophy, as powerful as hospitals and law firms. Imagine...