I know for a fact that they are not. They're presented with the performance and they laugh when they feel moved to do so. There is no LAUGH NOW sign that lights up. At the same time, they know they're being recorded and their laughter will be part of the finished product. So they'll cheerfully laugh for a second, third, etc. take if they thought it was funny the first time. In that sense it's artificial, but it's uncoerced.