US states control registration. Also older vehicles especially kei cars aren't nearly as polluting as one would think especially if only used for work rather than daily use, with mpg in the upper 50 to low 70s though fairly crap nox by comparison to modern 2020s cars.
They also encourage lower speeds which could be a plus in every way.
That scales with distance. If you're driving 30kph slower for 2 hours, it makes a difference. If you're heading 10-15km to the shop or the creamery, it's not so terrible. Speaking as someone who was a speed freak as a new driver, and sets my cruise at 78 in an 80 zone today...speed ain't everything. You wouldn't want to drive one of these kei vehicles on long trips, anyway.
They also encourage lower speeds which could be a plus in every way.