I've long had trouble with the word "ethics" - if only from my work using Emmanuel Levinas on ethics as first philosophy preceding ontology, and being told by some that that isn't ethics. I have a work ethic (god, yes), and I try to behave ethically, although it bites me on the ass half the time.
So an ethic of writing only positive reviews?
There's a whatever dunks your biscuit moment - your gaff, your rules. If you choose to publish a magazine, webpage, anthology, newspaper, blog or whatever, which only includes raves and never put-downs, then it's your dollar. He who pays the piper ... tells him to shut up and go elsewhere.
There used to be free movie magazine, probably called Flicks, which had capsule reviews, and hey, all of them were positive, even for 8 mm. My betting is it was paid for by cinemas and film companies, if only via advertising. So it's a marketing tool. I'd turn to it for information, but I doubt it would make me see a film (unless it interested me) or avoid one.
But let's assume it's not for mercenary reasons, that you want to enthuse and to encourage, that you have an ethics of evangelising, or pushing, and that you want to sell - no, too mercantile, you want to push... Buy this book rather than that book.
Here you'd run into the ethics as I would more commonly use it. I am reading a book and I want to give it a positive review - it's not by someone I know, I'm not being paid, I'm not furthering my career or feathering my nest... So, it would help if the book was, yanno, actually good. That makes the job easy in terms of writing a positive review - although depending on one's ability to rave it might become a bad review in terms of quality. The most negative response I've ever had was to a review which was meant to be positive - but in summarising the content I left the author thinking I thought they omitted that material.
But if the book is, well, yanno, not very good? You could accentuate the positives, you could misunderstand the point of the curate's egg metaphor, you can be mealy mouthed about "promising"s and "inherently interesting"s, but there's a slippage from economics with the truth and spin to downright lying. If you think Attack of the Clowns is a steaming pile of doodoo then any candour demands you should say so.
There are reviewers who do tend to the positive - the late KVB never wrote a negative review that I read. I could never work out whether he had low standards, only reviewed books he liked or - and this is what I suspect - he had the ability to fillet a book for what interested him. It did worry me though.
So let's assume good faith, assume you're not a paid advert - indeed, you are paying for the space - and you only write honest positive reviews. Or, if you don't like it, you decide not to review it. Or review it elsewhere. You are an enthusiast. After all, there are more sf and fantasy books published every year than you can hope to read, money is tight, and you want to know what to pick up from Amazon. You need a guru, right?
Rog Peyton and Justin Ackroyd both behave like that, handselling a particular book, and not just to pay their shop rents. You don't like the book, you won't trust their next recommendation.
But that there's the point. The taste of the reviewer is something you take on trust - and if they only write positive reviews then you might assume that anything they don't review is crap. But that's to assume you share their tastes. Tastes differ. One book I reviewed early on in a British edition went on to be shortlisted for awards in its US version, and people I respect talk it up (I can only assume it was rewritten). Of course, I might have faulty taste. But from what I like and what I don't like - and I hope I am consistent even if tastes can develop - you can calibrate how that fits with your tastes.
Here's another point: I might not buy a book that you like, because you like it. Equally, I might buy it because you don't.
This isn't just the fascination of the car crash - the turkey shoot - or the perversion of being single and bloodyminded (one academic I know seems to make a point of loving films others hate). This is the sense of knowing that tastes can become opposites. Christopher Tookey in the Daily Hate Mail and Barry Norman when he reviewed on the BBC are cases in point. Save for sf and horror films - which they tend to hate on principle and are often right - I find that the more stars they give a film the more likely I am to like it, and Tookey's turkeys tend to be the release that I'm looking forward to most.
But Pollyannas don't convince me. We live in a fallen world.
Of course, there are various reasons to read a book or seeing beyond it being good or enjoying it - but I suspect that that is what normal people do. I mean, we all went to see the sixth Star Wars movie suspecting it was going to be as bad as the previous three. But it was significant and needed to be seen by anyone wanting to understand (that part of) the field. Let's assume we're talking about the case of having a ten pound book token, and you want to know what to spend it on that you will enjoy (but even that word begs so many questions...).
Equally, I'm not convinced we need a Manifesto for Motherfuckers - and I don't want to stop anyone to do what they want to do with their spare time. But at the risk of turning into the how many sf fans it takes to change a lightbulb joke, it strikes me that if some of us weren't so enthusiastic about sf and fantasy, we wouldn't criticise it so much.