I havent tried the lending or borrowing from poloniex but I think thier investment with lending bitcoins to polonix users are secured with the altcoins stored in poloniex by the borrower. Just like in the lending section in this forum, collateral can be alts that borrowers dont want to let go.
The problem is that the collateral could drop 60% in value within a few seconds. The Poloniex might not have time to sell the altcoin and recover the bitcon for the lender.