Two reasons I can think of:
1. Infrastructure that's conducive to riding bicycles - on and off street bike paths, traffic lights with bike-accessable push buttons for 'walk' signs, maybe sensors in the roads to trigger traffic lights, mandatory bicycle parking requirements for new construction (commercial/residential), etc.
2. Marketing B.S. - Since Carlsbad is a coastal community dependent on tourist revenue, might as well take advantage of the designation for all its worth. Never hurts to promote that casual, easy-going, beachy lifestyle, including riding a bicycle (cruiser or other type) along the beach. Flaunt it f you've got it . . .