It isn't about lacking faith in a Man of Steel sequel, its about WB wanting to fast track the Justice League and do some cinematic world building so they can start making money off of more than just Batman and Superman.
Once they get that done they will make more Superman movies and Batman movies with Affleck and a movie for that skinny broad as Wonder Woman. and so one and so forth with other JL members.
whats interesting is that, if that earlier report/thread is true about marvel c-blocking fox but having a good working relationship with sony to the point where they may co-produce an Avengers 3 with Spiderman on the team... then I wouldn't be surprised if fox says fukk it and, assuming a good ROI on the upcoming fantastic four movie, decide to explore a F4/X-Men crossover flick 3-4 years from now.
The trend/direction now in these superhero films clearly seems to be team-ups and gang-ups, throwing as many faces in their without creating a cluster fukk; whether it be the avengers, or what they're tryna do with the sinister 6, or the x-men in general. DC sees this and probably figures that they don't got the same problem as marvel when it comes to multiple property owners, have just as many well known characters, yet they aint eating off anyone else but the same 2 characters. so yea, i def feel they got a "fuk this! we bout to jump
in this bytch!" vibe to them.
Avengers 3 w/ Spiderman. X-Men/F4 crossover. If any of these things happen or even if they dont and its just the ensemble cast of characters that Avengers and X-Men can bring in on their own, DC doesn't want to be on sidelines when all that is going down.