If you have a problem understanding how one harms themself by knowingly breaking the law and suffering the consequences that is something you need to resolve on your own.They've suffered harm. Perhpas that's the extent to which we can agree. If their suffering is the result of others disapproving of an adult ingesting stuff, I believe the harm is caused not by themselves, but by the others.
It's far simpler then that...the harm is caused by knowingly breaking the law.