I just don't agree that they are a tipped position unless they do something special or handle your bags. It's the same as a Disney bus driver, and you don't tip them.
I'm a very good tipper, and would be the first to extend a gratuity to someone who handled my bag or made my experience a little more enjoyable. But a driver who simply drives me from point A to point B, well like I said, a city bus driver does the same thing, and I don't tip them either...
IMHO bus drivers who don't handle bags are NOT in a service industry, they are drivers who are paid well over minimum wage, and as such there is no need to tip them.
Anne