Everyone has their own views of what is acceptable or unacceptable practice.
Personally, I have no problem with any review, as long as it's written from the heart, by someone with first hand experience with that provider.
Most "host rating" sites can have their ratings easily skewed both ways, for in fact, if one would want to knock a competitor down, one could do it quite easily, and inexpensively. I take it as a fact of life that the hosts that are ranked close to the top in the "rating systems" out there are quite likely actively working on improving one way or another their rankings there.
Hosts with lots of customers and already popular are likely to get some reviews pretty much anywhere, without somehow asking for them. A smaller host however (say having a couple of hundreds of customers) will probably get 1-2 reviews on a couple of sites. So, even if both hosts are equally good, only one will get grades in the ratings websites. It don't see this as entirely fair either.
Anyway, as a consequence of that, you'll see quite a few small (and not so small) hosts linking even from their homepage to one or two "hosts ratings" websites of their choice.
As long as there's no incentive for a (positive) review, no punishment for a negative one, and no punishment for not writing a review, I see it as a fair practice to advertise/publicize a ratings system, because the host exposes itself to be equally reviewed both by satisfied customers and unsatisfied customers. In fact, as we know, unhappy customers will be more vocal - it's part of the normal human behavior.