Single-lane roundabouts aren't very uncommon in America. Maybe not as common as in the UK, but they're definitely around and virtually every driver in America should be familiar with them. There will always be a few fools who don't grok them, but the same can be said for any aspect of driving.

I think when Americans say they're baffled by UK roundabouts, they're almost always talking about the huge multi-lane roundabouts. Those, to me, are nightmare fuel.