You leave, feeling good that you have purchased a bit of oral-pleasure, but the little man inside the vending machine knows better. You have taken my money and rewarded me with a hunk of chocolatey goodness. Come again, boy. I attract you with my gorgeous display, like a flower, or a species of insect, but I only serve to drain you of money, a little at a time. If I could, I would eat my own offspring.
Candy is inexpensive.
You eat fried styrofoam, and you pay for it. Paying for a filling treat from a vending machine is no different from dancing and pecking at the ground, while someone rewards you with sympathy money.