On the other hand, maybe this is really a lazy feature. It's probably a good idea for the system to disallow both incoming and outgoing network traffic to any program written in a non-memory-safe language that hasn't been signed in the past couple of years. The lazy version of this feature is just not to run any program not signed in the past couple of years.

Edit: Requiring a timestamped signature on the signature also makes it pretty easy to add auditing functionality to the timestamp server whereby the publisher can detect unauthorized signatures due to their private key being leaked/stolen by criminals or governments. If the timestamp server's logs show a signature by your key that you don't recognize, then something has gone wrong. On the attacker's side, they need to either steal the timestamp server's private key or publish their malicious signatures for scrutiny.