Unfortunately, I cannot find an actual source for this ban. So many sites repeat the claim, but with no citation. So now I am not even sure it's true. I tried searching NHTSA's website, but it's kind of a mess. I do know, that Citroën pulled out in 1974 from the US. The CX, released the year after, was never sold in the US, and the last year of the SM, which was popular in the US, only saw sales in Europe.