Look, it’s ridiculously simple.
The question isn’t, “why do people care that stuff was removed?”. It just isn’t, and it’s been answered multiple times anyway.
The question is: “why was that stuff removed?”
This is the question that needs answering. “Because” isn’t a valid answer. “They made, they own it, they can do whatever the fuck they want with it” isn’t a valid answer, either. You need reasons to do things that require time, effort, and dedication. Unless, of course, you’re crazy. An artist defiling their own work, if they’re not crazy, do it to send a message.
So: were the posters damaging anyone?
Did their presence make the game crash-prone, or bug it in any other significant way?
Did their presence violate some rule or copyright, making the game and its makers subject to legal repercussions?
If these are the reasons for their removal, or if there’s other any valid and compelling reason, why not state it officially to the public who payed for the product? Because if you don’t, then you have something to hide, and/or you do not care about your customers. Not a good look.
Given all of the above, it’s only obvious to think that the reasons for removing something so small and inconsequential had to be personal.
And what reasons might those be?
Looking at recent history, answering this is pretty close to adding two and two together.
The irony is, if the posters were removed by the will of self-proclaimed right-wing religious prudes, or if this happened in the age of Joe Lieberman and Night Trap, or if this was a Nintendo exclusive, all the “why do you care?” people in here would be unanimously condemning this.