Why should the entire flight be compensated? The airline provided the flight, just 24 hours later (and the delay is what the $100 are supposed to compensate).
Because the customer purchased a flight on a specific date, which was not delivered.
If you book a hotel room for 1-7 August to visit a conference, would you be okay if they said "oopsie, it'll be available for you 10-17 August, here's a $100 compensation"? What about a rental car, or food delivery? Is it okay for your wedding photographer to show up a day late? Of course not, the product you bought is inherently time-sensitive.
Many businesses have a notion of partial delivery. If you hire a contractor to build an extension on your house and they get sick halfway through the project and hand off to another contractor who completes it a few weeks or even months late, you don't get it for free.
