In America the Ultimate Edition of Batman vs Superman: Dawn of Justice has been rated R. Some believe this is to make more money by riding on the back of the success of Deadpool. I would like to point out that this version was handed to the MPAA for rating 5 months before the release of Deadpool so its release hasn't affected the release of BvS Ultimate Edition.
MPAA (Motion Picture Association of America) have rated the Ultimate Edition "R" because of "sequences of violence".
The MPAA severally restricts depiction of extreme violence. This can be seen when they threatened to rate avengers R for showing Phil Coulson being impaled on Loki's Sceptre.
In UK the rating company BBFC, They have decided to rate it 12 as the violence is not considered excessive. This highlights the difference in ideologies between the systems across the countries. The Americans are pretty stringent on violence and sex on screen. While countries like the UK and Germany are very lenient.