Because you made an agreement to pay someone back for the money you spent. You have a moral obligation to do so. If I give you money, and you don't pay me back, you've not only taken advantage of me, you've stolen from me.

It's not different just because it's a large institution. You agreed to the contract. You should uphold the contracts you sign.

If leveraged properly... This looks like a big credit card debt... Bad debt.