It's not prepaid, its secured.

It's exactly like requiring a security deposit on an apartment. You still have to pay rent every month, but the landlord holds onto some money in case you stop paying rent. In this case the lender holds onto some money in case you stop making payments.

You still have bills, you still must make payments on time, you still have the option of only making minimum payments and carrying a balance. The collateral you deposit is only returned when you close the account or the account is upgraded to a non-secured card.

It's not prepaid, it's a deposit. It usually gets refunded after you've gone some amount of time without missing a payment.