Must be a car service for foreigners run by foreigners. In these cases, it's common that American tourists tip them but tourists from other countries don't. Searching on the internet if tipping is expected in English is far more likely to lead to answers like it is "customary" than when searching in other languages because of all the Americans who insist on tipping even when it's actually not the norm.
Tipping in such cases contributes to creating an "American tax" when service people start expecting Americans to tip. This American tax is starting to to exist in Japan for guided tours and car services run by foreigners and specifically targeting foreigners. In other situations, a tip will likely make the recipient uncomfortable and/or confused.