Hello! I'm the founder. Your understanding of how Puter.js works is correct. We call it the "User-Pays Model" [1]: in short if you build your app using Puter.js, users cover their own infra cost, not just limited to AI. The idea is that infra cost will become essentially free for the developer.
As for user resistance, we haven't experienced that yet. I think generally for a user the puter.js experience (hopefully) beats the alternatives: 1) "give me your OpenAI API KEY to try my app" 2) Entering your credit card information in yet another app. Both of these also help apps as well, at the very least when it comes to conversion since you don't have to ask the user for their API key or credit card in many cases, not to mention you don't have to build metering, billing, rate-limiting, API key management, etc.
People use Puter for an incredibly wide range of things, including cloud storage, web hosting, coding, AI, and gaming. Right now, we're mostly focused on improving performance and making sure that it's as fast as a regular desktop environment!