I think an interesting question to ask at this point would be whether or not it's really meant to protect copyright, or if there is some ulterior motive for this sudden turnabout. For instance, if these Let's Plays are of actual in-game material, in essence, it's allowing a customer to 'window shop' in a way by seeing how the game plays.
The point that I'm trying to make there is that more and more triple-A games have been failing recently (according to publishers in determining production costs over sales revenue), with COD:Ghosts, Assassin's Creed IV, and others just not making up for their incredible production costs. I have to pose the question whether or not the fact that players can see into these games to make their initial buying decision, and seeing a game they like or dislike, will make decisions based not on marketing or official media, but on independent media such as Let's Plays. One must wonder if they (the publishers and developers) are worried that a look into their product beforehand may sway enough people to not buy their game, which would lose them money. However, it should be noted that by attacking Let's Plays, they either must have data, or only assumption, that the amount of revenue lost by unofficial media is equal or greater to that of the gape between their successful game and a financial failure.
Let's Plays also tend to have a good sway, with such prominent Let's Plays as PewDiePie, there are a lot of people who may have no intention of buying the game, yet still can experience the story and game play via Let's Plays. In this light, there is a point where some significant money is lost for publishers. When an individual may not have the proper console to play a specific game, a Let's Play video may instead allow someone to not buy the game, yet experience the story and mechanics, thereby allowing someone to enjoy the game without cost. The difference between Let's Plays and simply sharing a game is that a Let's Play will reach sometimes reach millions of people, while as an individual you'd be hard pressed to match that audience. In that respect, game makers/producers certainly have some real concern over the revenue that may be lost by Let's Play videos, yet I get the sense that most large Let's Play producers by now have some affiliation with a publisher or producer, and those that don't will end up having those videos removed, which, at least from my limited perspective, may negatively impact their immediate images, as well as only exacerbate the issues they're trying to circumvent.
With many gamers making buying decisions off of Let's Play videos, it could be added that the current expense of games is what drives the decision to not be as hasty as can be when buying a game. With games running up to if not more than $60 per game, the cost of the hobby/lifestyle is increasing as economic tensions show no sign of easing on those that have the means to make enough money to partake in the hobby, while those who are younger may be solely dependent on their guardian or parent for game-purchasing. In that sense, Let's Play's are vital for them to make purchases of games they find funny, entertaining, or compelling, and, with the move by publishers to remove Let's Plays and remove videos that do not directly benefit them, they may ultimately lose the younger audience for their games, some of whom my not be inclined to patronize those game designers future works after their favorite Let's Play-ers are effectively chained or destroyed.
It can be speculated that these efforts are likely happening now at the new current console generation because there is increased precedent for such struggles. With the arrival of more powerful (and arguably that's the extent of the difference) machines, and something so resounding and definitive as "new console generation", publishers may be attempting to get this business out of the way before the consoles move into major production, as an attempt like this during mid-generation would be far more devastating to them then were they to enact these rather draconian restrictions at the start of a new current-gen-era.
Ultimately, with the revenue lost due to lack of independent exposure, as well as goodwill being lost, and games continuing to fail to meet unrealistic sales marks, it would appear that this practice will only end up being a bad decision for game companies, and by shooting themselves in the feet like this to spite the ground they walk on will only make their already strained business practices more pronounced. While I have have hope that this decision will be swiftly reversed, I must also admit a semi-sadistic hope that these game companies will see their games continue to fail harsher and harsher as more and more Let's Plays are removed under the guise of copyright infringement.