> Why would companies know who's getting their products to review and why would reviewers know which products they're getting?
Amazon Vine reviewer here, for a handful of years, probably longer. They've changed how things are handled, so I'll say how it is now.
When I remember (they did away with an email of items you could select from years ago), I log into the Amazon Vine portal and see about ten things I can select to review. It seems to be somewhat based upon what I'm shopping for on Amazon, as more than once I've seen something in Vine a few days, weeks, or months after I've been looking for the item online (and sometimes have purchased something similar).
Anyway, I select the items I want to review and then get them within a week. We used to have 30 days (or four week, I forget which) to use and review the item, but now there's no date, at least not one that we can see. To review the item we go back into the portal, leave our review using essentially the same interface you use whenever you leave a review, and then move on.
When it displays on the site our reviews automatically get a badge that it's a Vine review. If the product ends up getting too many Vine reviews (or some other criteria) our reviews could be removed without notice.
So companies don't necessarily have a hand in deciding who gets their product, but I'm sure if they cared they could see who did.
A few times I got something just because, back when we didn't have to pay tax on the items we received, since Amazon could technically ask for them back at any time, as they still owned them, and my reviews were generally lower than my usual reviews because I didn't want the item to begin with.
Now that we need to pay tax I'm much more selective on what I want, and probably give more 4- to 5-star reviews because of it. If I just got something and had to review it, then it would be more likely I'd get something I didn't want (women's wear in a size my other half doesn't wear, or kid toys for a different gender or age range), and it's score would suffer.
From Amazon's perspective, I'm sure they rely upon the fact that reviews tend to be either overly positive or overly negative (when was the last time you left a review on something that was okay, or sufficient?) so that they can move product. Perhaps Vine helped with that. Perhaps their new program (the name of which I've forgotten) does too.
Personally, I can sleep at night because I'm okay with the scores I've given the items I've reviewed. Five-point scales are pretty easy; 1 = hate it, 2 = don't like it, 3 = no feelings either way, 4 = like it, 5 = love it. I've also gone back and changed scores because of what I've experienced later.
Back when they had forums there were clearly some people that just wanted the free products, and would then turn around and sell the item after a few weeks or months. I know I definitely wasn't one of those people.