The goal of AFM is to reduce pumping losses. Say that with me again: Pumping Losses. Cylinders are always trying to pull in as much air as possible and, except at wide open throttle, they are restricted by throttle plate. They want more air; the throttle says "No!" These are pumping losses. One of the reasons a diesel is more efficient: they throttle fuel, not air.
Imagine we have a large V8 lumbering down the highway. It's not working very hard. The cylinders are trying to draw a lot of air in, but they're working against a mostly-closed throttle. This is manifold vacuum. Now imagine that we halve the size of the engine. We still need the same amount of power, but engine is much smaller. How do we do it? We allow more air into each cylinder so that each cylinder can create twice the power it was making before. We do that by opening the throttle. This reduces pumping losses.
AFM effectively halves the size of the engine. Could we leave the valves open on the deactivated cylinders? Sure, but then each time air is pulled into or pushed out of one of the deactivated cylinders, we're going to experience pumping losses. So we keep the valves closed. The cylinders become air springs. Every time the air is compressed, it returns that energy on the expansion stroke. No pumping losses, only small friction and heat losses.
All of that to say...
The decompression system on a motorcycle is designed to reduce the compression in a cylinder to make it easier to turn over, either to save your leg or the flimsy little twig of metal that is the kick starter (or allows for a smaller starter motor.) It is not designed to save fuel. On motorcycles, they're inefficient fuel-wise because unburned fuel is pushed out the exhaust. If such a system is implemented with fuel cut off to the injector on our trucks, it would be inefficient because air would be needlessly pushed and pulled from the cylinder through the open exhaust valve: pumping losses.