Some of the characters make more sense than others when it comes to knowing what marijuana is:
Bugs Bunny knowing what a joint is isn't that crazy. HE'S AN ACTOR. On top of that, the Looney Tunes cartoon series isn't really meant for children (though there wasn't much in the way of drug references, besides the usual alcohol, tobacco, and prescription meds), so yeah, Bugs would know what marijuana is.
Michelangelo would know the most about drug related temptation.
Simon is The Smart Guy of the Chipmunks. He's supposed to have knowledge of surprising things.
ALF was once addicted to cotton in the live-action version of his show, which makes him act both high and drunk, so it would make sense for him to appear in this special. Also, the sitcom ALF was written and created by a Hollywood writer who was on drugs.
The reason why the toons were being such dicks to Mikey might be a parallel to drugs. At first, they seem to be helpful and good, but later down the line, they screw you over big time. It makes more sense if you accept the theory that the toons are hallucinations from the drugs.
Fridge Horror: The above brilliance is a double edged sword. At some extent, our classic childhood heroes know about drugs. Doubly so for Alvin and the Chipmunks if you know even the slightest thing about celebrity scandals.
Pooh staying behind makes sense when you consider he was the only toon present that's actually a toy, I.E. something Corey would need right now.
Likely unintentional, but despite the Values Dissonance, the toons are right, Michael can't buy weed since he's a minor.
It's not stated in the cartoon itself, but according to some advertisements Michael is 14 years old. And the flashback that Bugs Bunny shows him happened two years prior. Meaning that Michael started doing drugs when he was only 12 years old.
