It is one of the maxims of US media.

"No one in this world, so far as I know — and I have researched the records for years, and employed agents to help me — has ever lost money by underestimating the intelligence of the great masses of the plain people. Nor has anyone ever lost public office thereby."

H. L. Mencken, "Notes on Journalism"

Personally, I think that this rule being taken seriously has done more damage for US culture than just about anything else.