I suppose my AIBU would be that they should be treated the same as those that exceed the upper speed limit. Their poor driving causes others to drive recklessly.
YABU.
Firstly, someone else's slow driving is not an excuse for other drivers to behave recklessly. It can be frustrating, yes, but it's not an excuse for tailgating or doing dangerous overtaking.
Secondly, a lot of rural roads have the NSL as the default speed limit, regardless of whether it's safe to do 60mph or not. There's plenty of rural roads near me which are narrow, windy, frequently cluttered with tractors, horses, cyclists, pedestrians (no pavement on most of them), with very little visibility of what might be lurking around the next bend, and on roads like those, it would be dangerous to try and maintain speed near the upper speed limit of 60mph. Not to mention the single track roads which also have a default speed limit of 60mph.
I think there's a danger that penalizing drivers for driving at 40mph on good, fast, NSL roads, would also discourage drivers from driving more slowly on the kind of NSL roads where it's verging on suicidally / homicidally dangerous to try and drive at 60mph.