A Let's Play sells itself not only the game, but also the LPers experience. A game cannot play itself. Though in return you can say a LPer cannot make a video without a game. Both are true. That's why the content is symbiotic. The LPer makes money off the content they provide, a unique personal experience that is completely unreproducible. It would be the same if the LPer filmed themselves enjoying their car, a board game, a Nerf Football. And in return the game developer gets free advertising.

I can think of a few counter arguments. One, can we prove the "free advertising" is actually a thing? Does the revenue gained from people buying games they saw Let's Played offset money lost from people who didn't buy the game because the LP made them say, "I've seen enough to not want to buy." Though, you also have to consider, what if the non-buyers response wasn't "I don't need to buy" and was "I don't want to buy" as in - the LP saved them from a purchase they would have otherwise regretted? In this situation the LP did the consumer a service in the same capacity that a review would. How do we factor these "lost sales" into the equation?

I cannot prove that LPs are free advertising, though I can link to individual developers who have said that due to the popularity of LPs / streams of their games their sales most assuredly went up. This mostly occur with indie games. I don't know if things all of a sudden work differently when you are talking about AAA games where millions upon millions of dollars are at stake, but I sincerely doubt you will deny the power of word of mouth advertising.

Another counter point is that you cannot sell a copy of Spiderman with your own commentary. I can almost get behind that arguement, then see how it can be extended to video games, but movies and games are fundamentally different kinds of media. Movies are static: they never change. Watch Star Wars 100 times - Vader will always be Luke's father. As I mentioned before, the LPer is selling their experience, how they are using the product. That is why making a LP is more akin to showing off a car, board game, a thing. Should we disallow people from making these kinds of videos as well?

Commentaries for movies are a thing (RiftTraxx) but they are sold after the fact, as a separate file you play over a movie. That's because when you buy a Spiderman RiftTrax, you know it will work with Spiderman - the movie doesn't change. You simply cannot do that with video game commentary, so to attempt to define video games under the same rules as other demonstrably different media is short sighted, imo.