Usually because people remove it. However not having the laminar flow screen is better than having a severely damaged one. Point is for the AFM's karman pillar to generate the cleanest street possible the airflow striking it must be laminar. Look here:
http://en.wikipedia.org/wiki/Kármán_vortex_street
It's why this engine will often idle funky when poorly thought out things are done to the air box or filter. Don't expect a perfect idle without a filter or one that sits too close to the screen. Have at least one diameter upstream of the AFM so the flow can straighten. More if the screen is missing or the inlet is near the cooling fan. Especially if near the fan. I mean jeez, this is basic fluid dynamics.
Oil? Anything that obscures the surfaces of the emitter, mirror, or detector can result in bad signal. Anything that changes the mirror's mass can result in bad signal. Anything that results in a turbulent street can result in bad signal even if the electronics are good because, after all, the sensor is simply mirroring the pressure pulses of the street.
I was talking about the OP's problem though. He should stop screwing around and focus on what the diag system is telling him...