I got one good reason why not... Look at what happens with legal products that cause you bodily harm -- cigarettes. Trial lawyers and various other advocates decided that since these 'evil' tobacco companies make a ton of money off of a 'defective product' - they sued for (and won) a multi-billion dollar lawsuit. Now, since they've set the precedent, all types of substances that cause you bodily harm are now being targeted. They tried to sue gun manufacturers for making an 'unsafe product' -- apparently, the damn things kill people when you use them. Go figure. I guess the fact that the gun lobby is so strong and that the manufacturers that were being sued were also supplying the military with weapons, those cases got nixed. Well, now apparently there's a movement afoot to go after the beef industry, because since we fat Americans eat way too much red meat it's giving us heart disease and cancer. I guess since the cattle barons have manipulated the herds in such a way that we get decently marbled beef...well, sue them too. So, in effect...legalizing a product just adds more complexity to the issue. If some major corporations get involved in the growth/manufacturing/distribution of some highly potent drugs...you know consumer advocates would turn right around and sue them for making an 'addictive product'. Oh wait, didn't the tobacco companies get sued for supposedly making cigarettes more addictive? And you don't think weed could case lung cancer...
             what's prosecuting these crimes doing? For the most part, putting nonviolent felons in jail. In addition, due to the conspiracy laws related to drug trafficking, people unconnected to dealing can be imprisoned if they're associated with the dealer - say, roommates, girlfriends, etc. - and in some way, even unintentionally, aid his dealings (including paying rent, as it provides a location for the business).
             there are a lot of things legal now that slowly kill people, it's learnin...