trahs Hearp, It doesn't matter if you consider rosin and abraisive or not, USBC Rule Chapter 7, 1b2 says that "No foreign material may be placed on the outer surface of the ball."
Since rosin is a foreign material, therefor it cannot be left on the ball surface, you must wipe it off. If these bowlers are using it on the track area of the ball and not wiping it off, they are definately breaking the rules.