Don't worry about any confusion caused. I am just glad I now understand what you originally meant and can now resolve and comment about this autoplay issue.
Anything with a link to a YouTube video has the ability to open the YouTube application, in any core app at least. However, whoever posted the video on YouTube must have allowed the video to be viewed on mobile devices with a built in YouTube app, i.e. iPhone and iPod Touch.
Embedded YouTube videos on websites shouldn't be able to play directly on the webpage. They should have a thumbnail and a blue play button on it. Once the play button appears, you can tap it and it will open the YouTube app and automatically play the video. If you tap "Done" or come to the end of the video, the YouTube app won't be shown, the webpage will reappear and you will be back to surfing!
David