Earlier quoted context omitted.
They aren't just firing up a virtual machine on over-provisioned hardware, they're dedicating an entire bare metal machine to you. It is in their interests to make that worthwhile commercially.
It's entirely possible that the 24hr minimum is driven by the operational overhead, not just the licensing terms. Since this is bare metal, re-provisioning between customer sessions might require some level of human interaction, and this needs to be baked into the cost model of such a service.
From https://aws.amazon.com/ec2/instance-types/mac/#Pricing:
"Billing for EC2 Mac instances is per second with a 24-hour minimum allocation period to comply with the Apple macOS Software License Agreement"