The checkboxes are defaulted to be checked. If you want to remove this, look at the checkbox field code after applying the form to the page. Find the code that looks like this:
Remove that code, since it adds the code that specifies the checkbox is checked.
The reason the Same as Billing Address does not copy the information over is that the script to copy the billing information to the shipping information runs when the checkbox is checked, so if it is defaulted as checked, the information never gets copied over. the checkbox is meant to be checked only after the billing information is all filled out correctly.