Google tells you to use “nofollow” so that it will be just another
piece of advertisement or paid review, and not manipulating PR or link
popularity. That means per se Google is not against the ads or paid
reviews. In other words, persons selling ads, writing for money,
including bloggers, are free to sell their services as usual.

Review of products and services is a very normal thing, especially
in the print media. Many writers specialize in reviewing products and
services that are of interest to the readers. If it appears online,
there are a few things that may affect the site/blog in which it
appears. That is when you reference the source, usually another site or
sites where you can find such services. Good enough, from the readers’/
visitors’ point of view. But it is not so with search engines,
especially Google. Google will say that you are writing for money
(well, which journalist or creative writer does not want money?).
Google will say that it is unnatural because it will help the
referenced site to get a higher page rank (PR) and advises the
webmasters to desist from buying and selling links in this way. If you
do not listen, you will end up losing your PageRank.

Then what is the fuss about PR dropping and paid reviews? The
problem is NOT with Google or bloggers. The problem is with the site
owners/webmasters who hire your services who are buying links from you.
If they are in fact interested in advertising with you or paying you
for writing reviews, they should not object to “nofollow”. If they
object, they are trying to buy link popularity for pushing up their PR
unnaturally. Google penalizes the PR of both buyers and sellers of
links by lowering PageRank.