Most bizjets have very little room aboard, so they have to choose between lavatory seating and emergency lavatories located under normal seats. Emergency lavatories are a subject of some horror stories rivaling Stephen King himself.
So, a small bizjet lavatory is rarely the same thing as a restroom aboard a passenger jet. The usual idea is to try not to use it at all, except when the only alternative is worse.
The Hondajet is unusual in offering a luxury as lavish as a fully enclosed restroom in such small a size. Still, it's a luxury, competing with extra seating, with the expected result of getting jump seat certification for extra flexibility. You wouldn't use that seat for the owner or their guests, but rather for staff or cabin crew.
If a commercial toilet on an airliner gets equipped with a seat belt, that seat will also be used as a jump seat for the crew. Some part of the cabin is already spent on providing crew seats, so that would get redistributed to revenue seating, and crew moved to the "new" seats. Needless to say few are looking forward to it.