The trick is that your collateral is sold in free market if you are too greedy and the price of BTS falls down, like it is now. You loose money in that case.
Edit: No, bitUSD is US dollar derivative (a caltulated currency) that you can exchange to US dollars as well.
100% correct.
SmartCoins are guaranteed to be worth at least their face value. 1 bitUSD = 1$. Their value are backed by the BitShares core currency, BTS.
But if you want to convert bitUSD to real US Dollars (transfered to your bank account) : you have to convert it to another crypto. For now.