When you sell a few million copies of your game, maybe they will pay you to put their name back on the machine. Till then, better safe than sorry.
There is, of course, a very, very small chance that someone affiliated with Coca Cola would ever see the thing, never mind care about it. But what if your game sold a few hundred copies and by chance, turned out to be offensive to a certain group and the news picked it up. Imagine the news story with a clip of your offensive game play and in the middle of it all, a Coke machine.
Stranger things have happened.
I'm sorry, my answers are limited. You must ask the right question.