If you
have knowledge of chemistry nobody is trying to stop you from teaching it to
anyone you choose. If you happen to be a public school chemistry teacher
they most certainly do control whom you teach it to while on duty at the
public school.

I know how to fly [...] I am not however a CFI. Could I teach
you how to fly? Probably. Could I sign you off for a check ride? No. There
is nothing in that rule that stops me from teaching you what I know. It does
stop a CFI from using a privilege he has been granted by the US government
to certain people that government has decided it doesn't want to have that
information.

That's all well and good, but it certainly does not stop a CFI from (like the public school chemistry teacher) teaching on their own, with no logbook endorsement. They would not then be using the privilages of their CFI certificate.

In the end, it makes it harder for legitimate (but private) people from learning to fly, but doesn't stop terrorists at all. Terrorists don't need a logbook endorsement. They just need FlightSimulator 2004.

Jose