At first it was banned purportedly because it made people violent.

A few years later after the "Well, duh" discovery that people smoking it did not become violent, but instead became lethargic, the ban was upheld in the late 40's on testimony that it would make our men so peaceful that they wouldn't be inclined to fight communism.

A view which the 60's arguably stands as evidence for...

Cheers,
Ben