Why shouldn't B&N carry this material? It not in any way illegal to post information on how to grow marijuana, and quite frankly you might imagine a lot of the material to apply to many types of farming
Hemp is actually a substantial crop in canada and other parts of the world.
Your statement that B&N 'advocates' illicit drugs is quite inflammatory however. They arent 'advocating' growing/using/selling drugs any more than I am by discussing it in this post. They also carry such publications as 'Cosmo', which means they advocate materialistic, pleasure-your-man and buy-expensive-meaningless-crap women. Hell, they even have books with "OMG" naked women depicted in them. The horrors!