JSA already got it right.

I accept my failure and hope to grow in my deduction and reasoning. May the power of my belly and North Korean awesomeness vaporize you all!

It was pretty simple answer actually. I thunk too much about it. That always seems to be my problem :/ any who that was fun while it lasted. Felt like an Asian Sherlock Holems.

JSA's answer doesn't explain how the stand owner 'knew' that the guy wanted regular, only how he 'guessed' that the guy wanted regular.

For example, suppose his guess was wrong, and the guy intended to tip 25 cents, but absentmindedly forgot to state his order. Or suppose the guy had misread the sign and thought that diet was $1.00 and regular was $0.75, or that 'cola' was $1.00 and didn't realize that diet was cheaper; maybe he had forgotten his eyeglasses at the grocery store earlier that day. Or maybe the guy was paying with Canadian money, and he knew the exchange rate was $1.00 CAD = $0.75 USD, but the stand owner didn't recognize that the coins were Canadian and not American. Or maybe the guy was self-deluded and thought he could 'push' the idea that he wanted diet into the stand owner's mind via telepathy, despite being handed change to cover a regular cola, hence he didn't say his intended order; he was testing his abilities or trying to show off to his girlfriend who awkwardly overlooked his silly delusion because he was actually quite a nice guy deep down, and also fabulously rich. Or maybe ...

You get the idea. There are any number of strange circumstances which *could* have been the case. It's only by guessing that the *most likely* explanation was the *actual* explanation that the stand owner happened to be right. But if instead of saying "Thanks," the guy had responded with, "Oh, actually I wanted a diet cola. You see, it just so happens that ... blah blah blah..." then the stand owner would have realized that he didn't actually *know* the guy wanted regular, he had only *guessed* that.

Long story short: To get from 'I'm guessing that' to 'I know that', you need some extra information that shifts the probabilities from just 'probably' to 'almost certainly' (or, in some rare cases, 'absolutely certainly').
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It's probably because of the fact that the customer had change instead of a dollar.