The web-flow signing system is for users’ convenience in places where it’s not feasible to sign the commit with their own private key: commits made in the web interface or on an ephemeral GH-provisioned VM (codespace). For the latter, you are free to send your own private key to your codespace so you can sign your own commits but GitHub cannot because they don’t have your private key and don’t want to have it. Defaults matter and signed commits are important.
As a sibling notes, this use case and similar ones is the reason the committer field exists as distinct from the author field. I think a $10K bounty for this bug speaks to how seriously they stand behind the fact that they will only sign and mark as verified commits whose author field matches an authenticated user.
(Disclaimer: former GH employee)