Because yeah it sucks but Heroku is shutting down their free tier because of abuse so it's not a theoretical problem. Anyone who has even more lax requirements will get run over as well.
If you launch Fly from the CLI to test the free tier, you don't need a credit card. Obviously when you exceed the limits of the free tier, you'll need to provide a credit card.
Last time I tried fly.io I had to provide payment information before doing something useful. I'll give it another try as you suggested.
I realize that's what you're trying to avoid. In your case I'd recommend running `fly deploy` from your CLI If you don't want to leave the CLI. You could wire this up as a git hook either on your workstation or third-party git server. I recognize this isn't the same thing as Heroku, so I'm calling these work arounds :-)