[joshmoz]

Head of Let's Encrypt here.

You nailed it. It's important that our certs be free because we can't automate a billing interaction. If we had to charge then sysadmins couldn't just type a command and be on their way. Automated renewal could fail because billing info was out of date. This stuff has to just work, reliably, if we're going to expect the entire Web to use TLS.