I'm sure people semi-rarely drive them 24 hours straight. There are certainly situations where people do that outside of motoring events, driving several states away on a road trip for instance
(I have driven across the country several times, and if I had been doing it with a rental then I would have been inclined to maximize my hours-per-day to keep rental costs down). It would no doubt be rare, but I can only imagine that they don't get bent out of shape when it inevitably happens.
Basically, I hypothesis that rental car advertisement is far more honest than ISP advertisement. Furthermore, I assert that if rental car companies tried to pull the shenanigans that ISPs pull, they would be smacked down. An "unlimited miles" contract may have a "keep it legal" clause, which is a reasonable clause that anybody would expect, but a "'unlimited' means you can drive it during any time of the day, but keep it under 500 miles" tinyprinted clause would be considered deceptive.