I think people have a bit of a popular misconception about the Explorer advertising thing. What's happening is that MS is adding a banner to Explorer advertising paid OneDrive subscriptions.
I totally agree that it's massively inappropriate, but I do think that most people are imagining third-party advertising which is not what's happening - a lot of people might not consider the current situation to be an "ad" necessarily since it is pushing a subscription for OneDrive, a Microsoft product that you already have installed. It's more of an in-app purchase kind of offer.
If this is all it is, then it is not unprecedented. I remember that Windows 95 did something similar with MSN (which at that time was also a dialup service).
Yeah, I recently installed Windows 95 in a VM to run some old software and I was actually quite surprised at how hard it pushes MSN - the "connect to the internet" wizard it presents basically asks if you're on a LAN and, if you say no, tells you to call MSN to sign up.
As-is, I don't think it's a big deal. This is a slippery slope though. Look at what cable television has become. You pay for the product and they shove advertisements down your throat. Satellite radio is doing the same thing.
I totally agree that it's massively inappropriate, but I do think that most people are imagining third-party advertising which is not what's happening - a lot of people might not consider the current situation to be an "ad" necessarily since it is pushing a subscription for OneDrive, a Microsoft product that you already have installed. It's more of an in-app purchase kind of offer.