I fully believe you, bluefall, that that's why you do it. As do others who are mindful of waste. But I assume in such cases you and others either first ask, "Are you going to finish that?" or wait until the person has pushed the plate away or left the table or otherwise signalled they're done.
What I was talking about is people who jab a fork into your plate and take a huge gob of food, without asking, while you're still eating, and in some cases when you've just gotten your food and were just about to dig in. I've known people like that and yes, they usually do it with a big "Lookit me, isn't this cute" grin on their face. That isn't cute; it's rude. No, they're not "bad" people, just individuals who perhaps were never corrected when they picked up that habit.