The word diet comes from the base word for day. Your diet simply means WHATEVER food you eat in one
day. It also means everything you DO in one day, not just what you eat. To be correct, a food diet
for losing weight is always known as a "weight loss diet", but people just say diet as a
colloquialism.
And why do you associate a person's desire to eat LESS food with greed? You would think that
someone who did NOT want to eat less food was greedy.
Going on a diet does include the habit of eating healthier but it also includes exercise. It is the
whole package. I know when I say it, I mean that I am watching what I eat and working out daily. It
doesn't necessarily mean the person is greedy. It just means they want to lose weight or be
healthier.
I don't think greedy is a proper word to use. However, you are correct. The word diet refers to the
food that you are eating, not to eating to lose weight. A better word for weight loss, would be food
management.
Usually, "I'm going on a diet" means "an eating pattern aimed at losing weight". It may mean
something else but 99% of the time, it's a way to lose weight.