Well you're right most of the way, but you forget that caffeine is very easy to kick, just a headache and some attention problems are all the withdrawals I've every experienced.
If the government taxed and regulated marijuana, they would make shit loads of money. It may not be addictive, but its just to perfect to stop voluntarily. Seriously, if I could, I would stay high my entire life. It just makes everything that much more fun.
Depends on the person. As for nicotine, I smoked a pack a day for 5 years, then quit, no problems whatsoever. I drank coffee for 2 years and quit and for 2-3 weeks after that I was deathly ill, vomiting, etc. Every drug affects everyone differently, but for me caffeine was far more destructive than alcohol, tobacco, and weed put together.
I think Harry Anslinger can answer the OP's question best. The reason getting high is so wrong is because it makes white women desire relations with coloreds. It also makes the coloreds think they're as good as white men. Oh yeah, and it might make you like jazz music or some shit.