I've been reading An Army At Dawn: The War in Africa, 1942-1943 by Rick Atkinson; in the first few days of the fighting the capture of Casablanca from the Vichy French was planned by Patton. After the capitulation, and only hours before the attack was to commence, the bombers, already in mid-air, were called off. "A Wilcat flight leader radioed, 'Boys, it's all over for now. Let's go back.' The pilots jettisoned their bombs in the sea and returned to the carrier." Why were the bombs jettisoned? My only guess is that the greatly reduced payload saved fuel which was desperately needed or saved enough fuel that it made more sense per dollar.
Aircraft generally can take off with a heavier load than they can land with. This is especially true for carrier aircraft, as they tend to travel faster on landing. As the mission was cancelled early, the aircraft would not have burned off enough fuel to put them under the safe landing weight, and so would need to jettison their bombs to land. You also have safety concerns - if an aircraft crashes on landing, it's far safer for it not to have a bomb on board. Landing aircraft was still relatively risky at the time, especially on carriers, and given the possibility of heavy damage from an exploding bomb, jettisoning weapons before landing was common practice.
It was (and still is) a standard procedure for US aircraft carrier planes not to land while carrying live ordnance for reasons of safety. Not only did this lessen the weight of the aircraft, a major concern for carrier landings, but also minimized the damaged caused by accidental crashes. If the unarmed plane missed its arrestor hook and crashed into the ship or the crash barrier through damage, pilot error, or any other miscellany causes, a live bomb could add to the chaos and potentially endanger the whole ship. In the specific case of the Wildcat fighter cited above, the aircraft was notoriously tricky to land because of its narrow landing gear and there was a saying that there were two types of Wildcat pilots: those that have ground-looped their fighter, and those that will. Adding a live bomb was to this mix was a dangerous risk.
It was thought that Glenn Miller was killed by jettisoned bombs while he was flying over the English Channel, but this has since been proved unlikely: http://en.wikipedia.org/wiki/Glenn_Miller -see disappeance