Well it ain't coming from me
I think the only adverts I've ever seen on Chromecast were on YouTube and I don't watch it much due to the complete lack of interesting content. No adverts on Netflix or iPlayer and on the odd occasion we watch broadcast TV the misses mutes them all.
As for auto playing video adverts on the web, where are they? I don't see this phenomenon on any of the website I frequent, maybe its a Facebook thing. I refuse to be assimilated so I wouldn't know if it was.