Yes, @754, I think you've got. In the USA it was a tariff on anything over 750cc a.k.a. "heavyweight" motorcycles.
As usual, leave it to governments to stick it to the individuals. In this case, The Japanese did their thing in retaliation (or perhaps it was the other way around). They required anyone in Japan who wanted to ride a "heavyweight" motorcycle to take a special "safety" test. The test was absurd, and almost no one passed it. i.e. it was just a clever way to avoid saying "tariff" or "import restriction".
Interestingly enough, in Canada, up to the late 50s there wasn't even a license requirement for motorcycles. As "heavyweight" became an issue, licensing became mandatory (first it was only a census measure
yah right)
edit: TT was that the 1980s..man time flies. Could've swore tariffs happened earlier.
edit: rode my first motorcycle in 1956.