Yeah, its fairly harmless, no-one's getting hurt, and if he actually thought of it, he's sure as hell got the right to do it, and most of the staff just thought 'woo, it's a nutter' (tho the one that assumed he was gonna throw it shows how these people can be treated).
The only mild problem is if it becomes a genuine internet craze it'll stop being remotely amusing as thousands of people buy ice creams every day to do that. Course, it's still harmless and McD's will rake in a few thousand bucks a day extra.
The ideal thing would be if the staff knew about it and just flatly didn't react, the moment there's no reaction people would quit it.