That's actually a very common limitation worldwide for vehicles that have been imported into another country for tourism purposes.

Many years ago, I loaned my Canadian plated ST 1100 motorcycle to a German friend of mine to ride for a weekend in Germany. The German traffic police stopped him, and he had to pay full import duty (about €2,000) in order to not have the bike confiscated.

The rationale is that when a country permits a bona fide tourist to temporarily import their vehicle without fees, duties, or taxes, the importation is permitted only on the understanding that the tourist themself will be using the vehicle for touristic (non-commercial) purposes. If another person - especially a local citizen - is using the vehicle, then it is no longer being used for the purpose (tourism) that was claimed when the vehicle was admitted.

Having said that, if, for example, a group of tourists were riding together and simply swapped motos amongst themselves while travelling as a group, or if the rider who imported the bike swapped positions with the pillion passenger, I doubt very much if police or customs agents anywhere would object. The key issue seems to be that the owner/importer needs to be with the vehicle at all times when it is in use.

This comment addresses the question of "no third party drivers" from only the perspective of customs protocol. Whether or not the insurance someone has obtained permits a third party to operate the vehicle is another question altogether.

Michael