I have no desire to be hand-savvy so please ask whatever questions you have and I will do my best to clarify.
All of us were early employees of BrightRoll [1], an advertising marketplace that handled 10 million requests / second at peak. We've scaled large systems in the past and we've built this system with scale in mind.
Believe it or not, most workflows are extremely low volume. That, in part with the design of the system, allows us to offer the generous free tier.
We're planning to introduce paid tiers soon. You can see my comment on the parent thread that addresses some of the general questions around pricing asked by others in the thread.
This was a very early release of the platform and we hoped offering the product for free would encourage experimentation. To a great degree, we've seen that, so we think it was the right choice.