Before MoS, Cavill talks Superman/Batman crossover:
http://screenrant.com/henry-cavill-justice-league-batman-superman-man-steel-2/
Here, it shows discussions even BEFORE MoS have takin' place about following Man of Steel up with a Batman crossover and/or sequel to Man of Steel itself to introduce other characters through a MoS sequel, which is exactly what happened.
Then, this Zack Snyder interview promoting Man of Steel before theaters regarding a DCU
(6:00- 6:34)
https://www.youtube.com/watch?v=ExT5rqFqef0
It's all there, way before MoS hit theaters, you just got to read between the lines and look in the right spots. I don't think Batman is forced into it at all. I think this is what they set out to do in the first place and all they did was commit to it and actually DO it. Good job, WB/DC.