Max bit rate has been an issue since DVD days. When DVD encoding would spike the scene would have to be gone over frame by frame by hand to get it to look good. Maybe you blur a couple of frames before to empty out the encoder memory to fit upcomming frames. Have to decide which frames to lower resolution on that will cause the least impact, etc.
By using a higher peak bit rate available on BD they end up with fewer trouble spots. It really may not take much more disc space in the end (i.e. demand BD50 or BD25) since it is usually only frame sequences here and there, most of the movie is unchanged. The average movie bit rate may only move up a tiny percentage, just the frames that really needed it got it.