That's a good question, really. As to review whoring, that's something we don't put up with either. It's actually against content guidelines and whatnot.
I think that often, people discount constructive criticism, because they may be afraid if someone *gasp* speaks out, and actually DARES to say that this or that author misspelled, or the grammar is off, or whatever; the author will be discouraged. To my way of thinking, that's just silly, but there you are.
When I'm not up to my ears in other stuff, and take the time to read, I leave a review. Depending upon what I saw while reading, is what the review will have IN it. If something is so awful that I can find no way to leave anything that isn't an outright flame, I won't review at all.
If these people are honestly trying to improve their writing, concrit will help them do it. Mollycoddlying will not.