Because they use ONDCP statistics
That's the answer to the question Jack Shafer at Slate asks, namely " Why does drug reporting suck?" I'm not sure I agree with his point that the meth "epidemic" is being over-emphasized but I certainly would agree with that premise in the context of marijuana and no one could argue that this statement isn't spot on.
A great deal of drug reporting on [sic] major newspapers reflects ignorance, fear and false preconceptions.
One would say almost all of it would fall into that category, although unlike Shafer, I have higher hopes that reporters, or at least columnists are becoming more savvy about the issues. At a minimum they certainly seem to have become more outspoken on the failures of prohibition.