The primary goal of the U.S. Government is to stop illegal drug smuggling into the U.S., and their hard work has paid off. Most of the marijuana purchased in the U.S. is grown in the U.S.
They have been very successfull in cutting back the amount of imported narcotics, especially at our major sea ports like Port Everglades in South Florida.
Just because in your opinion it's a lost cause, doesn't mean they should stop. If they didn't try to stop as much as possible, we'd be a worse position. Because of their hard work and continue hard work, they've managed to clean up many neighborhoods and schools throughout this country. It's not a lost cause and the money used is well spent. They spend about 20 Billion (what's 20 Billion to the U.S? That's chump change compared to our other financial spendings that are not directed affecting this country) a year, arrest about 2 Million people per year for drug offenses, and this isn't including anyone arrested for marijuana, that # is around 700,000 to 1 million.
America's goal isn't to remove Marijuana. While they believe marijuana is a "gateway" drug to harder drugs like meth, coke, heroine and others, they do not feel that marijuana is that harmful, but it is still illegal. They want to rid our streets of that harder drugs. They also don't want you selling drugs. Just because you have marijuana doesn't get you arrested, if you have a small quanitity of it, they'll write you a ticket and you'll be on your way.
And finally, another major reason for our War on Drugs is the spread of diseases such as HIV. It's nasty to see crack users sharing needles, not caring what the last person has in their blood stream so they can enjoy an hour of being high.