I agree with you in theory, but in practice they are very different situations. For websites, the website owner loses traffic if they don't offer a secure experience -- they are in complete control of their destiny.
For email, the user loses deliverability and may not even be aware of it. Also it's often not within their control to fix. Marking non-signed emails as such would put a huge burden on email users and they might not even be aware of it.
For email, the user loses deliverability and may not even be aware of it. Also it's often not within their control to fix. Marking non-signed emails as such would put a huge burden on email users and they might not even be aware of it.