The ecu adds fuel as the AFM reads more air, if the ecu just smashed more fuel in all the time, the car would be rich (like this) when not under boost, then stoich when boost hit
You can have a turbocharged car with ecu and injectors designed to run boost, remove the turbo and it will run fine.
I know this is true, i've done it twice. One time to my ford Telstar TX5 turbo. It uses an air flow meter, just like nissan use. The only difference is the tx5 has one massive injector instead of smaller ones on each cylinder (like a carbie). The turbo blew an oil seal, i pulled turbo out, reconnected the afm and it runs fine, its just really slow and loud.
The second time to my z31 300zx. It was a VG30E, naturally aspirated. I had turbo injectors and a turbo ecu lying around, waiting to get a turbo. I had to pull the intake manifold off, so i changed injectors and ecu at the same time. The car runs exactly the same as it did before (misfiring, a little rich, hunts at idle, backfiring on overrun, it's always done this, its an '84 model car). Old injectors were ~180cc, new ones are 269cc (they sound tiny i know, but higher fuel pressure than RB25DE) and the ecu is calibrated to those injectors. We did exactly what i did to the zed, yet i got a completely different result, and my assumption was the fuel pressure regulator, as with the RB swap we kept the n/a fuel pressure reg, but on my VG30E I swapped the regulator with the turbo one (as i got a complete rail).
And now that an N/A ecu is running bigger Turbo injectors, when the turbo ecu couldn't run them... it doesn't make sense. If you put big injectors in the car goes really rich, anyone who has tuned a car should know this. You then have to shorten the injector times to get it to run stoich again, which is what the turbo ecu does.
Bah!