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.

It's trivially easy to run it straight for a single 24 solid hours (apart from gas and bathroom stops). Just take a cross-country road trip with 3-4 drivers who can rotate. Minneapolis, MN to Page, AZ for spring break as an example.

Technically you have to tell the car rental company if somebody else is going to be driving. What then happens depends on the company, though at least several rental companies will allow your spouse to also drive the car for no extra fee. Two people handing off every few hours can fairly easily do 24 hours straight safely (provided both are semi-decent at napping in the car). With three people or more, as you said, it becomes trivial.

Even in cases where the rental company charges you for additional drivers, I don't think that charge ever goes over a handful of dollars; not really enough to offset a 24-hour straight run.

From what I can tell, rental companies dislike multiple drivers because multiple drivers increases the risk of incident (I presume because multiple drivers correlates with 24-hour runs).