Arbitrage explains why prices tend to be the same across exchanges, but it doesnt explain why it isnt discounted today. Kraken is the only one where you trade tethers for dollars that I know of. You cant trade dollars for tethers on tether.to.

Its possible tether is able to sell tethers to exchanges alone and mint tokens at 1 on 1 rate and so it should tend to be so, but considering tether is issuing through bitfinex..