Their marginal cost for you using otherwise-unutilized capacity is near zero; probably just power really. So if it’s a choice between getting $0 and $0.1 for a $1/hr instance, that’s an easy choice.

You just have to be sure to make the spot stuff juuust unreliable enough that people can’t run their full workloads on it all the time.

if you've got the engineering in place for distributed fault tolerance you can run a lot of your workload on pre-emptable spot instances. There's an entire "as a service" product space that does this, abstracting away the challenging parts and arbitraging the difference

could you name a few such aaS service?