To detect the key, sure. But then there's the interface design to inform the user, hooking in to an email to notify the user if they're committing via git, a system to disable the option if necessary, monitoring the AWS format in case it changes, dealing with other formats that are requested by AWS's competitors, and so on.

I don't think this would be a minor undertaking or simple in any way, but it is important enough that a provider (and target) the size of GitHub should still do it.