Earlier quoted context omitted.
You think that's dumb, but I bet the marketing data behind that says otherwise. You think "I already have a mixer, idiot!" but in reality the chance that you are interested in buying a mixer just went from 1% to 2%. (Because you want another one, because you want to gift one, because you returned yours, etc.)
I think the problem here is that "recommendations that maximize expected marketing revenue" and "recommendations that maximize user experience" are not the same. Most of us would prefer Google to present us the latter, but they have considerable financial incentives to present the former instead.
That, I think, hits the nail on the head.
I think it explains the Amazon case quite well. I'm not so sure sure about the YouTube idiocy.