So what premium or interest rate does the system charge for this loan? Borrowers usually pay a premium or interest rate for borrowing money. Its more like lending to yourself and borrowing from yourself at the same time, instead of the system. As I said before, maybe lender is not the correct term to use. The smartcoin creator is more like an options seller.
In this case you borrow one type of money (i.e. bitUSD) against a collateral in the form of another type of money (i.e. BTS).
So one might say there is no premium or interest rate involved because the size of the collateral itself is enough to compensate the lender (i.e. the system) for their risk.
Lending to yourself and borrowing from yourself - this concept does not make sense. Similarly you cannot sell something to yourself or buy something from yourself.
In our case you borrow something (i.e. bitUSD) from the system and as long as you remain in control of this borrowed thing, you are not exposed to any risk (and any profit).
It's similar to borrowing money from a bank at zero interest and keeping this borrowed money safely in a bank account of the same bank.
Theoretically you have a debt but still your financial situation is not affected in any way as long as you do nothing with this borrowed money.