It's not a true ammeter that measures current flow. The shunt provides a known amount of resistance. The meter is connected to the negative and positive battery leads so voltage potential is known. It's also connected to both side of shunt to measure voltage drop across the shunt. By knowing voltage potential and voltage drop you can calculate amperage. 
It 'may' not work connected to the positive leads. Connected to the negative side voltage from the shunt leads would be close to zero volts, referenced to ground. If connected to the positive side leads from the shunt would be in the 12v range. That may confuse or even damage the meter.