It's risky, but it's also basically a free loan [...]
But here’s the pro: You can borrow money for free.

I don't get why it's called by many a free loan. You lock let's say BTS worth ~200 USD and you get bitUSD worth 100 USD.

If you buy bitUSD (if you need them) you would have 200, not 100 bitUSD. Thus you don't get any loan.

That’s right: 0% interest rate.
Try to find that anywhere else.

As explained above, you end up with less money at disposal than before. What is more, the interest rate is not 0% as you lend (well, kind of) 50% or more of your BTS for free the risk to lose a lot of them!
If you use as collateral your house, you can still live in the house and use borrowed money. It's not true in the case of BTS - bitUSD 'borrowing'.