Having spent time at college, I just have a gut feeling that if this poster were made widely available it would become an ubiquitous fixture in every stoner's dorm room across the country. I imagine it would have been especially popular during the 1960's and 70's when everybody would have been steeped in the sort of conservative cultural milieu that John Wayne represented and appreciated the juxtaposition of this icon of Americana and the hippie drug of choice. It has the same sort of appeal as the already popular "Nixon Bowling" and "Grow Hemp For the War" posters.
What makes it for me is the ambiguous nature of John Wayne's expression and body language. He could be fighting the evil dope peddlers and fiends, or all hopped up on the wacky weed, or, as I prefer to imagine, giving an arm-swingingly enthusiastic endorsement along the lines of: "Gee, golly! Marijuana sure is swell!!!"
Likewise, for those collegians who already possess refrigerator magnets of kitschy 1950's women with ironic captions (this demographic includeds everyone from party girls to nerds who want to assert that, yes, they too have a naughty side, thank you very much), many may enjoy this vintage movie poster:
I'm telling you, if these posters were sold wherever Scarface,John-Belushi-wearing-a-"College"-sweatshirt, Pink-Floyd-album-covers-on-the-backs-of-naked-ladies or Le Chat Noir posters are, then they would sell like hotcakes.