It is theft. Of a small amount, sure, but still theft.

A company giving you a Large because they ran out of Small cups, that's a bonus from them. But taking one yourself without permission, is stealing. "But it's 50 cents!" If a person went into a grocer's, and as a morning ritual, stole a KitKat, not because they were starving or broke, but just because they don't feel awake enough without shoplifting a chocolate bar every single morning... It's less than a dollar, but there's still something pretty important going wrong there, that isn't in line with the views of the company or society.