Airline responsibility in flight changes

According to the U.S. Department of Transportation website, the compensation maximum is $1,300 if the airline arranges substitute transportation arriving at your destination within more than two hours on a domestic itinerary or four hours on an international one. And you get to keep your original ticket and use it on a future flight. But, "If the airline must substitute a smaller plane for the one it originally planned to use, the carrier isn't required to pay people who are bumped as a result. In addition, on flights using aircraft with 30 through 60 passenger seats, compensation is not required if you were bumped due to safety-related aircraft weight or balance constraints." So US Airways wasn't required to pay you anything at all.