Surprised this sentiment is so common. It's like, do you want open source devs to work for free forever? Even Redis had to pivot to a business license.
I'm not sure what the terms are in this particular case, but in general, wanting someone to pay if they're deploying it to lots of customers seems reasonable.
On the one hand, in the same way any sort of fee (even $1) is a big impediment to adoption over free, so is the idea that "if I try this out and like it, I'll now be stuck with whatever costs they stick me with now and in the future". In a fast moving world, open source is just easier because if you change your mind, you haven't wasted money. In addition, there are only so many costs a project can afford and it is hard to know as it progresses where those will popup, so you avoid when you can.
That said, developers need to eat, and it is easy to appreciate the fact that they are letting you see the source, play with it, but pay if you want to use. I also fully support their right to license their software as they see fit.
I can swap one load balancer or cache for another. However, if my programming language has unfavourable terms I will have to rewrite all my code. Oh, and the knowledge I gained will be non transferable to another job or project because it'll always be a niche language. Better to spend the time learning how to get the same performance out of Numba or whatever other alternatives people mentioned here.
There are a lot of products released under GPL, for example, that make money for their authors. It's just that they don't make money with licensing fees.
I'm not sure what the terms are in this particular case, but in general, wanting someone to pay if they're deploying it to lots of customers seems reasonable.