The engine fires because the ECU doesn't use the AFM (whether it's connected or not) for starting due to Ks being unreliable at cranking speed. Furthermore a lack of Ks isn't detected until rpm crosses 500, the point at which it begins to be sampled. This is the same rpm at which the MIL is extinguished during normal operation after which, assuming no other problems, it remains out until rpm falls below 200.
Not all fault detection relies on the same parameters. There are some faults that will set on cranking only (CPS comes to mind) while others will set without any cranking at all. For example a disconnected AFM will set code 24 without cranking but, again, for 31 to be set rpm must exceed 500. It if doesn't get to 500 code 24 is all you're going to get regardless of how long the engine is cranked. Such a situation can lead one astray during diagnosis because, if the engine doesn't fire, the lack of Ks won't be sensed.
Others faults require a bunch of things to happen before they set. Certain O2 sensor codes, along with the knock sensor code, are the best examples.
