Aero bars are generally not allowed in mass start races, ie anything not a time trial, because they decrease maneuverability which makes them dangerous in the close quarters of the peloton. When the pros use them, they are not resting their backs; they are gettin' down and aero'. I have noticed the same thing, though. Most of the people I see down on the aero bars are tooling along so slowly even I could probably keep up with them. I assume they are just resting after a hard ride.