Ending S being the "best" ending vs. Ending A being the canon ending in the main game. The game is played from Chuck Green's POV, what with him being the main character, and ending S is the only ending in which he, Stacey, and Katey all get to go home together. So it only makes sense that this would be the best ending. However, in the process of making this happen he is forced to grab and retrieve a bunch of Phenotrans items lying around, which could mean the amount of evidence against them is reduced by some significant amount, in order to save them from the massively Ungrateful Bastard he saved from turning into a zombie. Ending S essentially requires that you bury a vital plot point for future installments of the series to expand on by letting Phenotrans get away with their crimes in order to give Chuck Greene his happy ending. Ending A, on the other hand, doesn't give Chuck his happy ending with Katey and Stacey. However, it's still a good ending, as the two girls escape safely. Furthermore, it becomes the lead-up to Case West, where Chuck ends up teaming with Frank West to gather significant evidence against Phenotrans, in the process discovering very real possibility of a lifelong/permanent cure for zombification as well as its link to the high heel face turnedSpicy Latina from the first Dead Rising. Not to mention Marian Mallon's threat of another harvest, which ranks under possible foreshadowing for Dead Rising 3. In essence, Ending S may be the ideal outcome for Chuck on a personal level, but it quietly lets the story recede, whereas Ending A plus Case West fundamentally continues the story.
The fact that some survivors are more concerned with things like poker games than the zombie apocalypse? It's done on purpose to make a statement about the mindless conformity and consumerism of modern life. Everyone who is a zombie in the game was already a zombie without the virus. When they were alive they were just a bunch of faces in the crowd, mindlessly consuming. It gets even more brilliant when you realize that the game takes place in a transparent in-universe stand-in for Las Vegas completely made up of shopping malls and casinos, the ultimate symbols of conformity and consumerism. Blogger Charles Phipps expands on this beautifully here.