Strange Initial Question
wasn't it? OP is at the beach, and he rents bikes. The proprietor warns to stay on the sidewalk because...why? He care about people? He doesn't want the insurance claim hassle? He's bossy?
Who cares? At the beach, ride wherever you feel safest. Try to be a good citizen. Don't damage the property of others. It's the beach, so have fun. Aren't these all anyone needs to know?