Because it's false advertising. This update sounds exactly like the usual "resolves issues" bulletins. If it would be in the optional category (or at least put up a confirmation screen on installation like some updates do) and if the description would clearly say what it is about (W10 announcer/downloader) then it would be ok.
But what they did is bullshit and it's on the playing field as all the other scammers. How can I trust Microsoft's "malware removal tools" and what not if they use the same slime-tactics when it fits them.
If something get's smuggled into the system because of a missleading description and puts up a nag screen, it's the textbook example of nagware.
So you've decided all by yourself that you can change the definition of malware. The next thing you're gonna tell us is that when users select "cancel" when the "upgrade to W10" prompt appears that Microsoft will send them an auto-response email saying they've locked all of the users pictures and won't give them back until they upgrade or pay Microsoft $1M.
Dude, get over yourself.