The prop cares. No idea why they didn't just program it to maintain 80% to a lower altitude.
First off, all that follows is SWAG based off a half-remembered Aerospace degree.
As other posters have implied, there a couple of possible reasons for this. Higher Np means more stress on the blades since the tips are spinning faster, but is better for the RGB since the planetary gear experience fewer stresses when operating near their designed constant speed of 2000 RPM (100% Np).
At lower altitudes, the speed of sound is lower due to temperature, you get more thrust at lower Np because of increased density, and the relative winds change faster .
The possibility of the tips of the propeller breaking the sound barrier is a real concern. The problem gets even more complicated when you consider that the propeller is not fixed pitch. So, it is probably just easier to use a courser pitch and lower Np at low altitudes and avoid turbulence or whatever causing enough change in relative wind to put the very tips of the blades in transonic regions, which completely ruins your efficiency and causes vibrations which in turn cause prop-sleeve touchdowns.
Also, even at flat pitch, 80% Np will result in non-negligable amounts of thrust if the density of the air is high enough. IE, you would still be producing significant thrust at Idle if you keep the prop spinning really fast.
So, to reduce wear-and-tear and make the RGB and engine more efficient, higher Np good. To prevent Prop-Sleeve touchdowns and losses in efficiency due to trans-sonic turbulance, higher Np bad.
So it is about trade-offs, wear and tear on the RGB, resulting in Chip Lights, or prop-sleeve touchdowns...Resulting in Chip Lights.... Above 10K, transonic tips are not such an issue at 2000RPM (Higher speed of sound), so you might as well increase the Np and save the RGB the stress and get fewer losses due to gearing.