You see, if you put down "Amazon.com
", it becomes an advertisement. If a company paid for it, it's fine (ie: Code Geass + Pizza Hut), *BUT* if you advertise something at your own will without being paid, then two problems arise:
1) Companies will start paying less for product placement and advertising because they can get free advertising from anime, TV shows, etc.
2) People will scream "NO FAIR, MALPRACTICE, etc" over it because you are promoting something over another "unfairly (bear with me, I'm just generalizing it into a 4chan post)".