A 1-star review saying it's completely overpriced, while not perfect, is still better than reviews I see in the app store like "Amazing app, love it! 1/5" or "Pretty cool app, but it crashes on load and I can't use it, 4/5" (and those are the intelligible ones).
If someone found something is poor value for money I'd rather know it before I buy. That kind of reviews saved me from a few bad purchases already.
I do agree there must be some part of "activist reviews" there too, not that they are worth the trouble of weeding them out.
This is especially apparent in certain categories, like Blu-rays, where people will arbitrarily use the star ratings to review either the content of the movie, the tech specs of the film print, the transfer from original stock to Blu-ray, the number of extras available, and so forth. It's gotten to the point where a star-based review of a movie, at a glance, is essentially meaningless. (Conversely, at least Netflix removes some of this problem due to its contextual purity).
The same thing applies, albeit to a different and lesser degree, with books. People use star ratings as referendums on the publisher, the author, the price, the cover design, the time of day, the weather, how many people showed up to their party last weekend, etc. There is no consistency in measurement. That's the nature of UGC, I suppose, for better or for worse.
If I had to guess, most people only click on items with high numbers of reviews, read the "top 3 most useful," and then make their purchase decision from those, rather than the long tail of rants from either the easily-bribed or disillusioned masses. These voters may inadvertently be steering people to the Kindle version by inflating the number of reviews associated with it over the hardcover.
I'm not arguing this is rational, just something I noticed myself doing - if others are the same, I can see a feedback loop where more reviews lead to more purchases.