It is covered in the rules - it says, anything not covered in the rules is down to the chief Marshall on the day.
Chief marshals are generally shooters with a lot of experience in the sport and are able to make a common sense decision without needing to have every conceivable situation covered by a rule.
If it's true that someone was using a rig to rangefind and one to shoot with then it would have been agreed by a CM on the day.
There might have been good reasons for it which the CM took into account when making his decision.
I've been a CM at a GP in the past and if someone rocked up with a positionals gun and a sitting gun - I would probably have told them it wasn't in the spirit of FT (depending on the circumstances of course).
"Rules are for the guidance of wise men and the obedience of fools" - Douglas Bader