It was probably due to a mixture of two things or one or the other.
1) Snake was included in Brawl, the ratings system in Japan may have gotten more lenient since that game's release (or DLC can be rated separately since its not part of the main game).
2) Joker's gun is an airsoft gun that just acts like a real gun in Palaces/Mementos.
That's a good point..I mean, he does,
That's it..he just stands there like that.
However, they could also pull from his artworks or his Persona Awakening pose for other Victory Screens (who knows, maybe he'll have a 4th Victory Animation used just for Team Battles).