but from nosy customers who would, without fail, feel the need to ask "WHY CANT YOU WALK?"

People actually ask that? :-O How exactly is it any of their business? And do they ask every single person they come across who is in a wheelchair the same question? Y'know what? I'd totally respond: "I can walk." And be all, "How dare you suggest I can't." Like, totally spaz out on them and make a scene just to embarrass them for asking such a personal question.
Ok, maybe in a work environment I'd just reply, "Because I was feeling lazy today."