You have to expect such ridiculous behavior sometimes, considering that even without roundabouts (which, as others have said, many US drivers just don't understand), some drivers will stop at an intersection and essentially insist that you go first, even when you have a stop sign and they don't. (I have had this happen numerous times and it's really frustrating, especially since the driver may or may not be making eye contact and waving signals, but usually I can't see through the glare on the windshield.)
Some drivers just don't understand that predictably following right-of-way (and other traffic) rules is best for *all* parties on the road. Yielding the right of way when the other driver is behaving aggressively or erratically is generally a good idea; but not when both drivers (riders) can be expected to behave themselves and keep traffic flowing along.