Frankly, I'm shocked the other major free providers (GitHub/Lab) haven't done this by default. GitHub's current default is free for public branches, and a small fee for private.
I could see a flipped setup working: by default a very small fee (1-5 cents per x number of batches) that most companies wouldn't notice, and a path for FOSS projects to apply for credits.
I can't find the direct link, but I remember someone on HN pointing out that because CI tools are turing complete, GitHub actions is the cheapest serverless cloud product in the world right now -- you just need to figure out how to game the system.
I'm sure they've built very sophisticated filtering tools, but imagine someone slips through the cracks and gets a cryptominer working. Get that action registered in enough projects (by, say embedding it in an Actions library or generator tool) and that could be significant.