We did go into this right back in 2012 when we launched - people asked then, too. No, we will not charge people for genuine developer use of imps.
TL;DR: we provide a really great industrial grade service that anyone can use to reliably and securely connect things to the internet. Many people who try this at home end up taking it into work because... well, it "just works" for years on end. Supporting developers for free proves how scalable our system actually is
Essentially, nobody becomes a commercial customer without using the developer service first, so it's a marketing/customer acquisition cost.
Given how many commercial devices are running impOS (1M+), the developer support cost is not significant,
We do understand that there are many developers who will never become commercial customers, but who have many tens of imps being used for personal purposes (I'm one of those people, with imps in plugtops, garden lights, environmental sensors, solar inverter monitoring, halloween setups, etc). We reserve the right to limit bad behavior - like running huge amounts of traffic through imps - but those people are generally aware they're being a PITA in those cases and take guidance well.
We have had "developers" who are obviously doing commercial deployments without paying us anything (hundreds of "developer" devices spread out around the world, or hundreds of vaguely identical accounts, all created by agreeing to the terms of service which explicitly deny that). We can easily find those people and talk to them about paying, though.