Quite simply every trailered coaster train needs to have a zero car out of necessity. On most newer trains these are located in the front of the train and the wheel assemblies are rigidly attached to the zero car frame. This is the reason for the phenomena you are seeing with some of the B&Ms. The connections between the zero and the "first" car are the same as with any other connection on the ride it is a difference in the attachment of the wheel assemblies to the car that causes the zero car to not arc away from the "first" car as the other cars do. The reason for this is so that the zero car hunts for the track, if the zero car was given the freedom in it's wheel assemblies the other cars have it would be a very rough ride. Now that does mean that riding on the zero car makes for a slightly rougher ride, simply due to the car always searching for the track. Now B&M has put seats on some of their zero cars simply because they have found out that they can. If the park does not want there to be seats there there will not be.