The rule could be simplified to make this the normal way ....
This is what it says:
The model shall be restrained at all times while the system is armed, either by the pilot (including holding the handle before, during, or after an official flight), an assistant, or mechanical restraint device (e.g. stooge).
Emphasis added.
It says, explicitly, that holding the handle after the flight is considered to be "restrained". There is nothing to interpret, it says it literally and it is unambiguous. If it is unambiguous, "interpretation" is not permitted, and I can't see how to make it any simpler. It was written that way on purpose to permit what appears to have happened. It's not tricky, it's not subtle (to be understood only after years of careful study), it's not intended to "catch" people over technicalities.
My guess is that they hold to a different standard at the NATS, based on the fact that before this rule was written into the rule book, they had an agreement about removing the safety thong before the airplane was disarmed. What appears to have happened is that they enforced this prior agreement. This is fine if that was made clear at the pilots meeting - which I admit I didn't pay much attention to. If that is the standard at the NATS, then do that at the NATS.
It doesn't form a "precedent", if not otherwise noted, the rule as written is the rule, it says holding the handle is OK, so it's OK. If that is not acceptable, then someone can modify it to include safety thongs or switching hands, and until then, add whatever you want to your sanction papers.
Brett