... but they've been doing exactly that for ages. Originally with link rel=prefetch, and later with ever more elaborate schemes which I think culminated with this: https://plus.google.com/+IlyaGrigorik/posts/ahSpGgohSDo
But prefetching based on giving hints to the browser has a bunch of problems. The most obvious one is the one hinted at here: you can have either something ineffective, or something that's effective but complex and not supported across all browsers.
> Not doing this is a strong hint that another agenda is at work, to say the least.
The sinister agenda of wanting things to actually work well.
Google CAN preload arbitrary websites. They fully load your website, JS included, when they index it. As for the security problem when on google.com, they could still preload the HTML, CSS, webfonts, do all the DNS/HTTPS overhead, all of which would be safe to do, save those first few seconds, and create a level playing field.