I was the office editor for a PR company, and a tremendous amount of copy hit my desk. “Disguising the agent of the action,” or my somewhat more pithy red ink note “By whom?” describes the problem perfectly.
When we’re writing to convey information, active voice is almost always more efficient and more effective. If you want to convey a mood, be as passive as you want.
Seems to me the real problem here is tools, like MSWord’s grammar checker, that don’t have on/off switches for various rules.
As the OP rightly says, there are situations where passive is more appropriate because it really isn’t about the agent. At the same time, there is passive and then there’s silly-passive. One where the agent is absent but implied and one where the agent is actively erased in a smokescreen of obfuscation. The former can be good, the latter almost never is. The “almost” being when writing a press release or political speech denying everything. 
You can turn off individual items in Word’s grammar check. It’s hidden behind a few menu clicks, so you might never see it if you don’t think to dig down a bit. But you can tell it to disregard passive voice, among other things.
But who would do that?
I would. I write intentionally. I don’t need “suggestions” from Microsoft regarding matters of style.
Word is pointing out some less-than-good habits I have. So I’m grateful for that even if it’s rather heavy-handed about it.
But yes, if I was churning out, e.g. documentation all day I’d be really annoyed about it complaining about our department’s already agreed “house style”. Nagging is never stylish coming from a computer.
As an ancient New Yorker cartoon once put it about a CRT + big desktop box old-school PC:
My computer beat me at chess.
But it was no match for me in kickboxing.
Yes I know. I was joking about the passive voice!
I’m the same. The only warnings I leave on in Word are for spelling. All the grammar stuff I find less than useful and more distracting than helpful. I’m confident in my grammar and style, and I’ll generally find errors or things I want to tidy up on re-read.
“Mistakes were made!!!”
Others have said mistakes were made.
The blame was shifted.
This is why I like the Dark Side philosophy of Star Wars.
“I am shifting the blame. Pray I don’t shift it any further.”
Refreshingly direct and actively voiced.
Ugh, bit off topic but nothing drives me battier than “some/others say.” Some say, huh? Name them.
Say that to half the newspaper articles these days.
Oh I quite agree w you. It’s the ultimate in saying nothing while casting carefully chosen suspicion, and using lots of words to do it.
I’ll never forget my shock, at age 17, when a college English professor at a dinner party my parents gave pointed out that some fantastic PR had gone into “The problem of battered women” (newly all over the news) Nope. The problem, he said, was “Men who batter.”