In terms of being a drug mule, if drugs were decriminalized, drug mules wouldn't exist, and most drug cartel activity wouldn't either.
People are advocating to decriminalize and allow people to harm themselves if they want, because it's their body and their choice. It's sad that people want to harm themselves, but ultimately, we do a poor job of stopping people, and by attempting to stop them, we've done considerable harm to our society (war on drugs, propping up cartels, etc.)
Which would also eliminate euthanasia as an option. If the law currently forbids euthanasia, then an "eat anything you want" law would still be invalidated when it's used for euthanasia, just like it would be invalidated for the other examples.
> if drugs were decriminalized, drug mules wouldn't exist
Cigarettes are not inherently criminalized, yet illegally selling cigarettes in bulk across state lines continues to be a problem. Illegal gun running also exists here, even for guns that aren't outlawed in any state. If there are cigarette mules and gun mules, there will probably be drug mules.
How about human flesh? Nuclear waste? Bombs? Bags of dangerous quantities of hard drugs [a la drug mule]? Endangered species?
Just like with free speech, "do anything you want for any reason" has the potential for abuse (both by a person to themselves and by other persons to other entities), and the majority of society is not comfortable with that. Sometimes we can be indoctrinated into tacitly accepting it after intense special interest lobbying, such as with handguns and cars. But there's probably no "greater good" aspect of being able to eat literally anything.