I agree that if a person is aware of your presence and asks not to be photographed you need to respect that wish.
It's also worth noting that the law regarding photographing people in public varies greatly from one country to the next. For example, in the U.S. it's almost always legal to take someone's picture in a public place whether they want you to or not. On the other hand, in France it's often required that you ask someone permission first, if they are the primary subject, i.e., not among a group of people in a park, etc. In Saudi Arabia, taking someone's photograph in public without permission under any circumstance will likely land you in very deep trouble.
But in all cases, common courtesy should prevail when the subject objects to having their picture taken.