I am not sure why site owners do this but having videos autorun is very annoying. I believe this detracts from a user’s experience with a website if they must take time to stop a video. I do not believe I am alone in being annoyed by this. Here are a few links that state how to stop autorun videos-
User Experience (or UX) will make or break a website. The only reason I could think why a site would want auto videos would be to inflate their video view statistics… which is then a false impression.
Please, site owners, do not allow videos to autoplay. It is not good UX.