In What Universe do Curfews Make Sense ...

... for preventing the spread of disease?

If preventing spread involves "social distancing" and not crowding as many people into a given place at a time, forcing businesses to close e.g. from 10pm to 6am daily achieves exactly the opposite of the desired effect.

Let's say a gym has 100 active members wanting to work out each day, and each member spends, on average, an hour at the gym (including clothing changes, showers, etc.). That means an average occupancy, at any given time, of about 4.2 members. 

If the gym is open 24 hours a day, at least some of those members will work out in the overnight hours -- meaning there will be fewer people, and a lower average density of people, in the gym at any given time.

Cut the operating hours to 16 and that average occupancy goes up to 6.25 members (ceteris paribus -- some people will presumably give up working out if the hours they want aren't available, and the real occupancy is probably higher during those other 16 hours anyway, but there will be some occupancy increase effect).

Ditto bars and restaurants.

Sure, there probably won't be as many people working out, drinking out, or dining out at 3am as at, say, 7pm, but not allowing people to work out, drink out, or dine out at 3am will just result in at least some of those people making those facilities more crowded at those other times.