How is a USD-chained currency like Tether economically possible?
Bitfinex, the issuer of Tether, claims that all of the USDT is backed by USD and USD equivalents. As such, the intrinsic value will be close to [$1 * asset impairment], and so no market manipulation/pegging is required to maintain parity, so long as their assets actually match the tether outstanding.

Although no action is required, there is an opportunity for Bitfinex to buy USDT with USD when the price dips below 100 cents and sell when its over 100 cents if they actually are fully backed. This would tighten the trading range, and make Bitfinex a profit in the process.