Because it's not up to the browser when the video is embedded using Flash?
That's up to the browser, too. Doesn't Firefox already block Flash from auto playing, or is that one of my add ons that I forgot about? Every time I go to a site with Flash, there's a big gray square with a play button in the middle of it. Good for Firefox, I say.
Autoplay does have some legitimate, non-spammy uses.