Schrade,
Let me try and clarify two points.
1. ECM grounds 4 injectors at a time because they are wired in parallel - TRUE
Jeff's comments are correct. 12 volts is supplied to each bank via a fused circuit. INJ1 is bank one with 4 injectors that uses a 7.5 amp fuse because it also provides power to the telltale assembly. INJ2 is the second bank of 4 injectors which uses a 5 amp fuse. So battery voltage is provided to each injector. One voltage supply is wired to each injector in parallel. The ECM injector wire is the return line that completes the parallel wiring. This allows any 1 injector (or more) to be unplugged on either bank yet still maintain injector power and ECM switching to those still connected. The ECM provides the electronic switch to ground at the bank level. The PWM is programmed pulses and durations based on GM's specifications. Think of this as an ultra fast relay with no mechanical parts.
2. ECM has built in protection from too much current - TRUE
When everything is working properly, that is with four 16 ohm injectors in parallel from a 12 volt source, the ECM 's internal electronic grounding switch can operate. Based on ohms law of I = V / R (Current = Voltage Divided by Resistance) if the supply remains the same (V), and resistance levels drop (R), then more current flows every time the ECM pulses a bank of injectors. If too much current flows we would overheat the ECM. The electronics within the ECM that is used to fire the injector banks can only operate with a specific voltage and current range. So this is what creates a built in safety net for the ECM that can shut down pulsing. We may see a no start condition (or other issue), but at least the ECM is not fried because too much current was flowing. In reality, the ECM has no clue what the resistance is of each bank of injectors. It works as long as voltage and current are within range. However, we like using resistance because it is easy for us to measure and pinpoint issues.
Comments...
Injectors should be very close in resistance, this is especially important on banked injection systems like ours. Flow balanced injectors are of value as well. You don't want any one injector causing a lean or rich condition when the ECM can only manage a bank of 4 injectors at a time. Just unplug one injector on the bank with the O2 sensor (driver's side) and watch what the ECM does. It sees a lean condition and increases PWM, trying to compensate. We only have one O2 sensor and it plays a critical role in PWM for both banks. If you apply ohm's law to parallel banked injector wiring, it doesn't take much drop in injector resistance to force the ECM out of usable range. What amazes me is how well banked injection actual performs.
hnoes . 
