It's a tech blog thing. The FTC decided all those free hardware 'raves' were a little shady and should be disclosed, and declined to give a minimum dollar value for gifts or free stuff that needed to be disclosed. It's been pretty common for tech folks to review hardware and either give it back or keep it and it was pretty understood they hadn't bought the product to those familiar, but.. on the side of protecting the consumer, they decided it was necessary to disclose. It's carried over onto the book folks from there... So that's where the rule came from.