I think going into legality is a bit too far, it's not really in the spirit of the game to go 'due to highway code 97882, this car is unavailible to be driven on american roads'. It's just not fun
Basically, the thinking behind it is about seeing a car where you don't normally see one, going back to the mustang example, like for like, in reality it costs about the same as an MX5 (which is common as muck) does over here, however, because you can't get a mustang without importing it, seeing it on the road in britain your jaw drops... which means that cruising around Ibiza in an imported stang would be a real head turner, particularly for people under level 10 who haven't unlocked Oahu yet
Island exclusive cars, and cheaper cars are also a really good reward when you reach Oahu... Yes, you get a whole island, but lots of people are just going to look at it and say 'been there, done that'. offering them different dealers, and cheaper cars is going to prompt people to explore the re-designed island