It helps to remember OE tuning priority. 1) Emissions, 2) Economy, 3) Warranty, and 4) Driver experience. $42 has a PE delay factor and many of the truck cals had a large delay which was only bypassed if RPM went above 4500 or so such as during a hard downshift. Fuel economy may be part of the PE delay factor but emissions compliance and catalytic converter operation are major players. When the AFR gets rich HC and CO increase, O2 decreases, and the cat tends to coold down and stop working. Lower than normal AFR plus a non reacting cat meant bad marks at the EPA emissions test center. The code and calibration are specifically written and adjusted to allow WOT operation during testing without going into open loop or cooling the cat.

Don't think GM isn't above fooling with the EPA. The reason highway lean cruise mode wasn't enabled in these pcm's is that cals were originally set up to stay out of lean cruise during EPA pollution testing but would enter lean mode during mileage testing and real world driving. Once the EPA realized what was up GM got into trouble for having an emissions defeating device installed and ended up disabling it in all subsequent cals where the code is present.

As an aside, it's worth noting that cat overemp protection code can prevent continuous operation of lean cruise mode. If the model determines cat temp exceeds safe limits it can (depending on the code) force 14.7:1 AFR and / or adjust timing to cool the converter and prevent meltdown.

Remember: Emissions, Economy, Warranty, Driver Experience.