Think of it this way. Let's say there's an intermittent speed sensor/conductor plate issue, right? The fault occurs momentarily then corrects itself, ok? The transmission will go into limp mode. If the problem isn't currently present at the next key cycle the transmission works normally again until the fault re-occurs.

This makes sense from a design standpoint. It wouldn't make sense to disable the vehicle because of an intermittent problem and FWIW this is a very common scenario. Hope this explanation helps