Unfortunately, it's copyright law. It's a legally binding contract that they agree to when buying a game. I will admit it's a problem for a lot of people who make a living off of it (and I'm often amazed they make a living off of doing this), but for indie reviewers to not make money off of their reviews seems like game companies fiddling with the data. I think the contracts/laws need to be changed.
Think about it. Many game review companies (such as IGN, Kotaku, and the like) have gotten into trouble for questionable deals in exchange for being able to review a new game, and a lot of them (after that whole Gerstmann/Kane and Lynch scandal) are afraid to be brutally honest about a game, even when it deserves it.
"You don't like this game? Well guess what? We won't give you any of our games to review anymore!"
Now, these people who do reviews on YouTube are people who bought the game with their own money (usually). Their opinions are brutal, honest, and untainted by the fear of being able to keep their job. They obviously care about games as a whole and put time and effort into their videos (...usually). They don't have marketing departments and advertising deals with major companies.
Game companies see this and, of course, they don't like it. They don't have any control over these reviewers. They didn't give them a review copy of the game, they can't call their supervisor, they can't raise a fuss and get them fired, and they can't just ban them from having any more copies of the games. So they do the next best thing: they bring in the law. They make contracts so long and full of legalese, you don't know what you're agreeing to without hiring an attorney to translate.
I'm not trying to be paranoid, but I'm seeing some rather alarming trends here that I'm not too fond of.