I would have thought the issue would have been covered in the Congressional inquiry and subsequent report.
Merely having an air defence system doesn't mean it works properly, and it would have been geared towards the former Soviet Union in any case. It isn't true to state that it was SOP for decades to intercept deviating or non-responsive aircraft. The resources to carry out this taskin the US would be immense and monitoring would depend on the prior country of origin or other information about the aircraft, which would (presumably) be continually monitored from reaching US airspace or radar cover. The system was not designed for or capable of dealing with this threat.