The reason a tank in the park violates the rule is because a tank is a vehicle and the rule is "no vehicles in the park".
The tank violates the rule and it is why it isn't allowed.
However there is a scenario (such as the war memorial,or an ambulance) where you may want the vehicle to have access.
I was just relating it to people who claim to want to "cut red tape/remove regulations" because most of the time the red tape scenarios are these where we want to be able to carve out exceptions for particular scenarios. But we still don't want it to be the default position.