Bitshares kind of hacks its way around this. If your collateral ratio ever falls below 1.75, you get put on the Margin Call list; the account with the lowest collateral ratio on this list immediately gets its entire short position put on an order book to buy back BitUSD at a 10% premium. For small drops in the BTS price this is fine; it effectively penalizes the accounts with the lowest collateralization, and serves to reduce the supply of BitUSD.
For large drops in the BTS price, this can be a complete catastrophe. If a short position is ever undercollateralized (collateral ratio <1), BitUSD has a stroke and shuts down, immediately closing all short positions and leaving only a little BTS in a shared collateral pool.