The reply below about using Firefox seems to be the answer from the viewer perspective. From UStream's perspective they need to monetize the bandwidth and hosting costs they endure. I liken free streams to free television. Free or super low cost television as in basic cable has to be ad supported. So I put up with it as a trade off against paying the content creator directly.
Streaming is still evolving and it is certain that new models will be tried that balance user experience with hosting provider's needs. I am just grateful that we have it at all. The amount of high quality pool available for viewing for free is astounding.
True but they could do something smart like: somehow put the ad at the top or at the bottom off the viewing area. They'll get a hell of a lot more eye contact that way. With the current set up does anyone not instantly extinguish the ad? If it wasn't interfering with what someone wanted to watch it could be left on longer and have an exponentially greater chance of being viewed.