Per the FAA the ONLY items allowed to be carried in the cabin of ExpressJet ERJ are personal items. They utilize a lower average passenger weight that doesn't account for bags carried into the cabin. On say a CRJ-700 or 900 operated by ExpressJet (be it in the Delta world) the average passenger weights are higher because carry-on bags are allowed in the cabin on those aircraft for ExpressJet. It all has to do with how the carrier develops its approved weight and balance program within the Operations Specifications. Because the weight of a carry-on is NOT included in the passenger weight on a 145, they must be gate checked. This ensures their weight is accounted for separately. Consistent enforcement is difficult BUT if it's not a personal item it should not be brought into the cabin. Consider that the weight and balance of the aircraft is contingent on proper adherence to this policy.