After they found out they felt tricked.
Most people probably don't ever know
Perhaps in the next version, each Pro Review can include a brief disclaimer at its beginning stating that it is sponsored by the company being written about.
I know when I click onto a page that has sponsored material, I like to know.
It's the right thing to do
The FTC has also updated their guidelines which state:
the financial arrangements between some bloggers and advertisers may be apparent to industry insiders, but not to everyone else who reads a blog. Under the law, an act or practice is deceptive if it misleads