Simple because most of these "Review" sites are provided by those hosting companies like Hostgator, Justhost and others.
Not all hosting companies own web hosting reviews sites. Not that getting listed on most of them is all that hard, or can't be solved with a bit of cash, unfortunately.
The titles were similar to the examples below:
Get Paid for Sharing Your Hostvwxyz Experience (and he goes on to say, "I am willing to pay $3 for a very short review (5 - 10 sentences)."
One month later he ran the next request.
Share Your Hostvwxyz Experience - $5 per review (And once again he offers,"I am willing to pay $5 for a very short review (5 - 10 sentences).")
I don't really see something wrong with that as long as the reviews are genuine and all reviews, positive or negative, are published. No different to pay per post on forums, or paying writers for articles on a given subject.
There are lots of people with valuable web hosting experiences worth sharing that need a small push to take the time and tell their story. If $5 is what it takes, that's fine by me. We all know that customers tend to get vocal when bad stuff happens, but not so much so when thing are going great. This sort of push may help get some of those positive experiences out into the light.
Now, you might not like the "Hostvwzyz" limitation, but there's a ROI to be considered. A fair reviews site that doesn't fulfill its owner's needs as well as those of visitors, will disappear, and the loss will be everybody's.
Just as if WHT, or Hostingdiscussion, or any other forum would fail and disspear -- we all end up poorer because of it. And just because advertising on these forums is prohibitive to some hosts, doesn't mean they're doing anything wrong.
A "top hosts" or reviews site will usually be willing to add any hosting company that has a well paying affiliate program. No one stops a host from offering such.