Why? Well I'm struggling to see what other purpose a "Review" would have. It's a critical appraisal performed by an authoritative source to help his/her audience decide on whether to purchase/attend something. I'm not sure that is a subjective opinion by me, perhaps it would be easier if you told what you believe a review should be for.

I would argue a review is essentially an assessment of a piece or work. The problem I am having with your definition, is that your adding a purpose to it that is not required nor inherently part of it, that is the goal of a review should be informing for the sake of a specific action.

As I said above, I believe a review should help it's readers decide if something is worthy of their participation. If not, then why and if so, why. A good review helps the audience make that decision and bad one doesn't.

But a review doesn't require that, and to further that argument, what informs one may not inform another. Simply look through this thread and find people who enjoyed the review because its the kind of review they wanted. I still don't believe at all that this is the purpose of a review, but if we are to subscribe to your definition, does it not do that?

I think that's a separate argument. That isn't related to the idea that you put forward that the purpose of a review is subjective. Of course subjective opinion is involved in reviews, that's fine... actually expected, but the overarching goal is help readers make an informed decision on whether to purchase something or not.

My point in bringing this up was that providing an objective standard on what a review should do outside of providing insight, is ironic due to the subjective nature of a review.

That's the same there is no wrong way to write an essay or a letter, where we all know that there are objectively bad letters/essay/literature etc. because of a multitude of reasons (grammar, spelling, confusing sentences etc.).

There is no real wrong way to write a letter or essay, there are just accepted ways to do so. What makes a letter or essay is not its formating, if you look up a definition of what an essay is for example there is no mention to formatting as a required element, just that its a short story. Plus, I mean, we are talking to each other in English, literally THE language of exceptions when it comes to grammar and other specifics.

If a story can be judged for being poor because it doesn't engage the reader, a review should be judged to be poor because it doesn't inform it's audience.

But it does inform, thats the issue, it just doesn't inform you in the way you have come to expect or want. Just as a book may not engage you, it may engage someone else. Just as a painting may not inspire you, you may not inspire anyone else. For example, I hate modern art (rusty paint can I am looking at you), I don't care for it at all and personally think its far to pretentious. I do however love surrealism as an art form, as it inspires me and brings forth emotions. This does not make modern art not art, it just makes it bad to me.