I mean this in the nicest way possible, but think about all the substances people legally put into their bodies that has the potential to destroy their bodies - cigarettes, alcohol, and others that alter their mood - prozac, xanax, caffeine, etc. I just like to point out whenever possible that cannabis is much less harmful to people (physically & socially) than many legal substances. California will have it on their ballot this November, whereas they will decide if they should decriminalise and tax it. It would be regulated much like alcohol.
If you are really interested in why it was originally criminalized, I can post that also, but did not want to get too off topic.
As far as these parents, I believe they were irresponsible, but the plastic bag it was likely in was more dangerous to him than the cannabis in it.