The Case Against Legalizing Marijuana

Why is the media unabashedly promoting the legalization of marijuana yet wringing its hands over the increase in heroin use, at least for the first news cycle after Phillip Seymour Hoffman died with a heroin needle in his vein?

Is it that heroin is that much more dangerous? In fact, many, many more lives have been ruined as the result of marijuana addiction. And in countries in which pot became legalized, use increased dramatically. For example, after marijuana became legal in the Netherlands, use among 18-20 year olds nearly tripled.
And lest you think I overestimate pot's dangers, see THIS.
The last thing the U.S. student body and workforce needs is to be less motivated and more memory-impaired and prone to serious cardiovascular disease, cancer, and vehicle accidents and the resultant mayhem and death. 
But you claim, "Legalization will eliminate the black market." Think again, dude. Already in Colorado, because marijuana is taxed at 29 percent, a black market is already burgeoning, in which you can buy pot  for 29% less. And as with all sin taxes, you can bet the tax rate will only go upward from 29%. For example, in most states, the taxes (federal, state, and sales) on a pack of cigarettes exceeds 100%!.) Of course, the higher the tax rate on pot, the bigger the black market. So all legalization does is expand the number of places that people can buy your memory-impairing, motivation-killing, cancer/heart-disease-causing drug.  
But what about for medicinal use? Marijuana is available in capsule form by prescription. No need to make it available through "pot dispensaries." Everyone knows the "medical marijuana" argument is a sham.
I've heard potheads then think they've got me by asking, "Hey dude, would you outlaw alcohol?" Yes, I would. Alcohol causes harm wildly in excess of its benefits. Of course, the perfect is the enemy of the good, and a black market would arise just as in Prohibition but, net, there would be less use and thus less devastation of humankind.
Our Socratic pothead may then try, "Next you're going to want to outlaw ice cream?!"  The risk/benefit of ice cream is much better than for pot. 
What about the libertarian argument that people should be able to do what they want as long as it doesn't harm others? Well, pot harms everyone: the user, his or her family, employer, the drivers and pedestrians who are in accidents because of the slow-reflexed, often glacial-speed driving pothead. How often do you see people driving extremely slowly on the freeway? Yes, some of them are old people who should have their licenses revoked. Others are new drivers. But I'd guess that a lot of them are zoning out while high on pot and other drugs. 
And then there's the opportunity cost. If pot and alcohol were not as readily available, more people, although certainly not everyone, would be more likely to pursue one of the infinite number of more rewarding, less harmful recreations: from learning to writing to mentoring to painting to befriending.
Pot should be illegal and if our legislators cared about us, they'd make alcohol and tobacco illegal too.
Care to disagree?