Glad the market doesn't care. I do think that this could unfold in a way that might be a thorn in Tesla's side. Reading through the NHTSA letter again now, and the inflection point appears to be, "NHTSA noted that the law says automakers must issue a recall 'when they determine vehicles or equipment they produced contain defects related to motor vehicle safety or do not comply with an applicable motor vehicle safety standard.' " A recall doesn't necessarily mean that Tesla has to bring cars into the Service Center, but they do have to file paperwork with the NHTSA and label it a "recall."
What is interesting to me is what standard gets applied here. I don't recall the numbers but I think there was a post some time ago with the number of accidents that occur where a car hits an emergency vehicle, where the car causing the accident is not using any type of AP software - and it was quite large as one would expect. So, then the question becomes, is Tesla's solution more or less safe than what exists w/o the software. If it is more safe, then how would that be classed as a defect related to motor vehicle safety...? What's the benchmark that is used to gauge this.
And getting this right with NHTSA is important as they could take punitive actions. It's not that different than living with rules that exist in China - Tesla chose to comply rather than fight. In the US there is more wiggle room, and maybe Tesla does need to fight.
I started reading the actual Safety Act, but I'm not an attorney and my eyes are glazing over - so I will defer to others on this forum who have more expertise in the law for their views.