This is what I think that more logical than "they don't have a reason to...". This missile is really cross over the Japanese land. If we look at their record on how they react on Chinese incursion to their ADIZ, it's really not logical. If JDSF always over react on intercepting Chinese Aircraft on their ADIZ, they should be more hysterical on how their react to a missile that cross their main island. This missile should be intercepted as soon as possible when they come to their land. Because if not, it just become a show of weakness on Japan prowess.
So I think that there are several probability on why JDSF not react on this missile
1. They want to raise the N. Korea threat to the next level. So they want to act like a victim who was threatened by a villain.
2. They react, but their Anti Missile is failed. So, in order to hide this humiliation, they act like that they don't react at all.
3. They don't have any Anti Missile can that intercept this IRBM. Just like what you read at the report. This missile is at 770 km altitude. So maybe JDSF doesn't have any Anti Missile that can shoot this IRBM because of the range. Maybe it is within the range of SM-3. But maybe there is no SM-3 at Hokaido.
4. They are too late to react. They don't have enough time to counter this IRBM when it approach Hokaido. So there is weakness on their detection capability.
Maybe there are other probabilities that I haven't think of, so you can add it here.
