As a developer, I get upset when a user writes a negative review, sometimes even a very comprehensive one with several paragraphs detailing every step of whatever, blaming me for something that in the end has nothing to do with the add-on they’re reviewing (this happens more often than you’d think).
The review guidelines are open to some interpretation after all. For example, riddle me this, would you consider this to be a helpful review:
1 star - “Meh…”
Meh? Why meh? What’s meh? Does it not work? Is it not useful in your case? Does it look bad? What/when/where/how/who…? Does a review like that deserve to be shown with just the same full value as all other reviews to every single user viewing the page?
The structural problem you mention has already been recognized: a lack of qualification system for the reviews themselves. Which is basically saying this: currently all reviews are valued equally, and presented to all users as such. In those conditions, I definitely don’t want unhelpful reviews scaring away potential users, especially not just because “they took the time to write them”.
I’m not saying that all of those Ghostery reviews should have been deleted. If I was part of the AMO moderators team, I probably would have kept a few of those in the screenshots, but definitely not all. But I believe the ability to flag and remove unhelpful reviews or those otherwise not-valid is necessary until such a qualification system is put in place. And specific cases like these should be handled as what they are, specific cases, not as an excuse to blame and take down the whole system.