I believe Tether is backed by centralized reserves. So though you can access it on the blockchain, if their fiat reserves were seized/confiscated by a government or other they'd have a problem. (Whoever has access to the Tether funds could also possibly run away with the money at some point.)

BitUSD is decentralized, it and the backing for it exists on the BTS blockchain so .gov can't seize/confiscate your BitUSD or the collateral that backs it.