It's not a question of the law, silliemille/joe, it's as B00 says. We used to train our petrol station folk not to react for that reason. Sometimes it went disastrously wrong.
One time in favour of the operator was in Belgium - on the motorway from Ostend to Brussels. Gang appear in a service station near Bruges, in front of one of those "cashier booths" - and with a shotgun.
"Hand over your money"
"No"
Bang..................................................and that much shot came forward, hit the windo (which was bullet proof) and hit the gang on the rebound. Didn't take the Belgian police too long to fine leaking criminals.
The corollary of this was a gang in the US - same scenario - except that the cashier, unbeknown to management was armed. From inside the boot, when challenged, he produced his pistol and fired - the bullet ricocheting around and killing him.