They probably use the AWS Activate program (which my main startup is part of). It basically gives you a set amount of "credit" with AWS. We got ours almost a year ago and it just ran out this month. It allowed us to run key workloads on AWS for a year at zero out of pocket cost.

Of course Amazon does this, in part, because once you have built a functional business on their platform(s) you are more likely to stay there and to continue paying them long-term.