Corporate "business" rates, while higher may come with a kickback at EOY to the corporation. Leisure rates wouldn't have that kickback. Basically UA is giving that credit to the employee directly. Also, businesses are committing to a certain amount of corporate spend and if people used the leisure option that wouldn't count toward the required spend, which would erode the discount and impact the kickback. Important to book business rate for business trips and leisure rate for personal trips.