My guess is that these are isolated converters. So the input and output are not galvanically connected. This is typically required for radio transmitter equipment to prevent ground loops. This is where you can get all kinds of funny effects because of impedances of equipement relative to the minus/ground lead. Another example is NEMA-2000 supply that (officially) requires a isolated ground as well, so for this you need to use an isolated converter.
It may be they found a problem with some devices not having a ground connected to the battery ground and therefore had to connect the two 0V lines together.