Marijuana might make someone great at building model airplanes, and it has a lot of life-saving medicinal benefits, but driving a car while high is a different story. Marijuana is absolutely a safer alternative to alcohol health-wise, but that doesn't make it acceptable (or legal) for users to drive while intoxicated.
In New Zealand, officials don't want to see a cavalier attitude about recreational marijuana lead to intoxicated driving and crashes or arrests. Don't get me wrong: I firmly support the legalization of marijuana in America. But if we're not careful to discourage driving stoned, it might become a big problem.