In pretty much all of Europe e-bikes with a throttle are considered a type of mofa/motorcycle (depends on top speed) and come with all kinds of additional rules, such as needing insurance, mandatory helmets, having to drive on the road, license plate requirement and at type of motorcycle driving license for the fast ones.
Here in the UK, only pedelecs are exempt from road licensing laws, so the throttle bikes aren't legal to use on public roads. Additionally, they have to stop providing assistance at 15.5mph or so. However, it's common to see riders illegally using throttle based bikes (e-motorbikes really) on the roads.
I think the "most" is overstated; I've been shopping for a year or two, and most of the ones I see have throttles.