PCB Battery Grounding
The point of the battery protection IC is to disconnect the battery when it's over-discharged. It does that in your schematic by disconnecting the BATT- terminal from the rest of the circuit using those MOSFETs as a switch. 

An alternative way of doing it would be to disconnect the BATT+ terminal from the rest of the circuit. But it is usually easier to disconnect the negative side if you are using MOSFETs, because it needs a N-type MOSFET. High side switching needs a P-type MOSFET, and they are more expensive and/or have less good performance.

When the protection circuit disconnects the battery, it is important that it is disconnected from everything (other than the protection IC itself). Anything still connected could draw current, and damage the battery. So every other part of your circuit should be connected to GND, not to BATT-.