If the billing address and the ship to address have to be the same, why would they bother having you enter the same info twice? Once should be enough, agree?
Are you sure it isn't an optional button you could hit that asks if the shipping address is the same, and fills in the info if you click it? I see that a lot when I order on line.
Could be fraud protection, could be the type of merchandise, could be the dollar amount, could be your country laws.