I don't even see where there is a problem with this. People who do Lets Plays are providing entertainment and more importantly free advertisement for the specific game, and when it's someone like PewdiePie who has such a large audience and influence, revenue should not even be considered a problem.
I know if I was a game designer, i'd be begging PewDiePie to make a Let's Play of the game I made, not banning him and others.