Again, no rule for it in real world, no need to have a rule in IF.
Why shouldn’t you go supersonic in a area that is for supersonic training in real world.
Take it vice versa, we need to enforce special use airspace. It is pretty annoying when you have to stop your BFM cause some stupid commercial or GA pilot didn’t respect your training area airspace.
There are several training areas over land where fighters go low level at high speeds in real world.