I think this is configurable on the streamer's end. I watch a lot of PirateSoftware and he's talked about how choosing to run ads at certain intervals vs running them before the stream starts affected his viewership levels. I don't immediately have a video of him saying that, but I imagine there's a YouTube short where he describes that.
All things considered though, I think you are right that the option that forces it at the beginning of the stream is obnoxious and affects the interest of those intending to stream. There's likely a better way (similar to your suggestion) that gets people involved in the stream first.