They'ed be paid if they were on the premises and doing things their employer permitted. Generally they'ed be required to clock out if they wanted to say take a smoke break, or run some personal errands, or take a nap. Hell they won't even generally get paid for the time they spend commuting to work and similar activities that are in practice necessary for the job but during which they are not at the employer's command.
There are jobs where you need warm bodies and you are paying for availability. If you hire a security guard, you're hoping you never need them to do their job, but you're paying them to be there in case they are needed. If a gig employer demanded your time regardless of whether or not you were doing a gig, of course they should be paying for your time. But a big part of gig work is that when you're not in the middle of a gig, you have no obligations - you can sit around waiting for the next job, or you can do something else.