Don't plates of food carry implicit permission? One person wouldn't bring a plate full of brownies if she intended it only for herself.
And if she did she would make a note saying so.
And if she did she would make a note saying so.
Of the 15 or so roommates I've had in my life, 13 or 14 would have almost always answered "yes" to the question "hey, can I take XXX from the fridge and eat it." Whereas fewer of them would have answered the same if I had asked for money. Therefore, the decision to take something from another person is heavily influenced by the anticipated response of that person (and therefore not as illogical as the researcher is concluding).
"I'm sure he won't mind if I just take a few dollars," seems more wrong than "I'm sure he won't mind if I drink one of his fancy beers/soymilk smoothies/etc".
The researcher in this study is saying "aha! taking a drink is exactly the same as taking money, therefore human beings are illogical thinkers in this scenario!" But it is in fact logical - it's based on the anticipated response of the person whose money/drink you're taking (and if you don't know how it belongs to - the general expectations of people in your society/community).
Also, I'm sure that there are societies in which people never take drinks from other people, and consider it the same as taking money. A comprehensive study would have covered more than an American college campus.