Placing a shunt to monitor current in the negative lead is not always possible. To monitor a battery, it really does not matter if the shunt is placed in the positive or negative lead. But as shunts take on a more general metering role, finding a place to break the negative connection isn't as easy. For example, monitoring the current from an alternator that makes it's negative connection to the engine block makes a shunt in the negative lead impossible. DC-DC converters/chargers that don't have separate negative connections (non-isolated) are another example of where a shunt must be in the positive lead. Many times, loads are returned to the vehicle chassis rather than a separate wire to the negative side of the battery. This is another example where the shunt must be in the positive connection to that load.