A course designer creates a shooting position and marks it either with a hoop or square to ensure that everybody not only shoots the same targets but does so from level, safe, ground.
Just think what would happen if there were no marked stand or safety cage from which to shoot… competitors would simply end up standing where they liked with no consideration for anyone’s safety!
In fact, depending on the terrain and the type of target being thrown it is sometimes possible to create a bit of an advantage for yourself by taking up a non-central stance within the shooting position.
If such an advantage does exist then good on you for spotting it but don’t push your luck by placing a foot on the hoop to make even more of the opportunity.
In a situation such as this the referee did the right thing to point out the error of your ways and give you a reprimand.
The only time you can fairly argue the point is if the stand, for whatever reason, doesn’t afford you a safe, secure, stance from which to shoot.
In such a situation the ref will stop shooting until the under foot conditions have been fixed satisfactorily by ground staff.