They aren't the author so why should it matter?  The story is pretty much written before they see it, so I don't see any conflict there.

Not unethical. I have done so with most of my writers. The editor/beta can only see the final product when it is posted, so in effect, it is "new". I really don't have a problem with that at all. In many cases the Beta or editor can see things in the published version that they missed whilst going over the work. Its all good.

No conflict. The editor or beta is a reader, after all.

I've "liked" pieces I edited or beta-read, and left reviews, both. At times, it's important to me to state publicly how I feel about a piece. I've also sent private reviews to an author, telling the person how I see their work progressing as a whole.

I asked the same question.  In my opinion, if you want to review the finished product for you author, I find nothing wrong with it.  It's also a sign of support to the one you are working with:)

Unless you are making the good comments along the way - "i love that line"  or " good job describing that" etc, its  important to put your positive comments somewhere and a review is the best place for them!  Only hearing about problems or what your editor doesn't like would be a real downer.