Why do you think content owners would see this as IP theft rather than free advertising? Are people watching play throughs rather than playing it themselves?
As one less recent example, watching Half Life 2 with funny commentary is great, but I have no desire to ever play the game myself.
How many of them would have been so pushed as to play the game for themselves anyway... an open question.
I watched Let's Plays of both because I wanted to see the story, and because the players were entertaining. I don't think I'd have enough time or patience to play through either game, but I definitely got value out of the game itself without having played it.
It does seem like a gray zone - I experienced content without paying for it, but I didn't experience it the way it was truly meant to be. Did I come by it unfairly?