I have a suspicion that this is again one of the treasures we owe to the lawyers.

I'm not so sure. In the good old days (i.e, C19, of course) lawyer's draughtsmen were paid so much per word for legal documents, so they would have been more likely to write, "those who own a car or a bike or both a car and a bike are supposed to know traffic rules," thus increasing their emolument by seven units times the rate.