Supposing it was a foreign missile one explanation for not firing on it might be that we saw it was headed away from CONUS immediately drew the conclusion that it wasn't a threat and let it go.
The best speculation I have read is that one of our subs had a leaking missile and had to unload it fast. If we see anomalies that hint at failure in flight that fits. The PR embarrassment of an unintended launch of a hot missile on our coastline. Bad timing for 0bambi if that's all it was. lol
A hot missile that had to be aborted? I would think it would have had a self destruct after so much altitude. If it was an errant shot from a boomer then if anything I would think they either want it vaporized or easily recovered.
The theory of a super sonic tube launched or vertical launched cruise missile certainly is interesting. was it a test, or was it a message? Or both?