Any company offering an API is going to have some percentage of their customers every month who are shocked to learn that a bug or other failure to rate-limit their own scripts has led to a bill that's 100x larger than they were expecting.

These customers are angry, call Support and Accounting to demand refunds, and cancel their service. There's no upside to having unhappy customers.

Switching to pre-paid billing is probably just better for everyone.

Then why aren't they accepting new customers