Look at it this way. If I buy something from a store and pay in cash, and then the cashier takes some money from the register and hands me the change, that's okay. But if I open the register myself and take the money, they call the police.

i.e. just because it's POSSIBLE to do something doesn't mean it's okay to do it.

Well, as long as you don't steal anything or assault the cashier you haven't done anything illegal (even if the police is called).

You're example fails.