Oh, absolutely. I'd even expect them to be whitelisted by default (although that would pose the interesting problem of deciding which sites deserve the privilege of being whitelisted out of the box).
Unfortunately if there is a "default" whitelist, it will become a "whoever pays $x money gets on the defaulted list." (I'm not referring to only Apple but if Chrome, Firefox, etc come out with the feature).
Not to mention it still seems much more user friendly to make this opt-in rather than opt-out.
Youtube autoplay is the devil. If you want to open some videos in several tabs, you have to individually pause every tab. It's a real pain. I'd much prefer it if Youtube autoplayed only when you switch over to the tab.
It already does behave like that for me. Definitely in Safari but I think in Chrome too.