Yep, but BTC is still traded for money, not potatoes or cabbages. So tax will be eventually applied, soon or later.
It is for now. But what happens when there is no need to exchange Bitcoin for money to buy things. Bitpay has over 10000 merchants that accept Bitcoin directly, this number is growing daily.
http://bitcoinmagazine.com/7023/bitpay-surpasses-10000-merchants/Imagine when you can buy things from the local store with Bitcoin. Until Bitcoin is recognized legally as a currency, all trades done with Bitcoin are to be considered bartered trades, no tax applies.