Because this is about the TV-Show and not everyone has played the games. If you want the talk about the games without the spoiler tags go to the game threads. It's the same with any adaptations. No matter if you played the game, read the books, the comic books, manga, etc. You always by courtesy at least put spoiler tags on things that have not happened yet in the show, when discussing the show.
If people who have not seen the show go to game/manga/comic books/whatever thread elsewhere and get spoiled about the show that is ongoing, then it becomes their fault. But here, no. The people that are in the know should be the ones just making a very small [ ] on their posts