I think it's Hempseed rather than weed itself, but proper Nanami/shichimi togarashi (7 spice blend, roughly translated) has Hempseed. Because of the ban on drugs and such, most exports and US versions exclude it though.
As an example, this site shows and advertises it's usage, then on the same page says:
Hemp seeds match sesame in terms of flavor and aroma. There is an old Japanese saying: "Those able to mill hemp seed are truly mature."
No hemp seeds are used in products for overseas markets.
I think it isn't illegal if it can't grow a plant, so if heated or ground. You can buy ground hempseed, and i think i have some downstairs. Still, imports and such are much trickier, which is probably why they don't use it for exports. Also other countries may have stricter imports, and perhaps the company doesn't want the headache of verifying which market to send which product to.
It's similar to how most companies just conform to Californian restrictions rather than make different products for different states in the US.