They may be looking at a correlation between speed and claims rather than whether or not the speed is legal. Accidents at 80 mph will tend to be more severe, and possibly also more frequent.

Note that they are also looking at night driving, which as far as I know is legal everywhere, but someone who spends a higher percentage of their time driving at night probably is a bigger risk for the insurance company than a similar person who doesn't drive as much at night.

I'd surmise it's because several states (CA comes to the top of mind) set speeds in excess of 80 as a potential felony enhancement.

iirc in CA it's 20mph over the speed limit, or speeds over 80.

The insurance companies probably want to know who to raise rates on.

It's not arbitrary, there are limits of physics to how fast you can slow down on rubber wheels no matter how good your brakes are. The stopping distance starts to grow dramatically around these speeds.

Even it it's legal, it's probably less safe. Insurance companies care about your likelihood of being in an accident that they will have to pay for, not strictly whether your driving is legal.

people that drive on those high speed roads are more likely to be involved in more dangerous/harmful collisions?

legally and unlikely-to-make-expensive-consequences are separate items that insurance exists to differentiate

why shouldnt people driving on such dangerous roads have to pay higher insurance rates?

It's not arbitrary. In Virginia that's a guaranteed reckless driving charge.

In jurisdictions where 65MPH is the highway speed limit, 80MPH is usually the "reckless driving" threshold. And in Virginia, reckless driving is a felony misdemeanor.