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.
Bitcoin it will never be treated at the same level as fiat money. Therefore there will be always necessary to exchange BTC for any fiat currency. That is what Bitpay does, it help merchants to receive fiat money after an on-the-fly exchange for BTC.
You are failing to understand that BTC never will be legally recognized as fiat currency because the BTC supply cannot be controlled by any central bank.
By the way, BTC is a medium of exchange used primarily to buy and sell fiat currency. This has nothing to do with barter. Everyone in need of BTC needs to pay for that with fiat currency. Even if someone accepts cabbages and potatoes to sell BTC, at some point that BTC will be exchanged by fiat currency.
If you think that tax is not applied when trades are done with BTC, you are completely misinformed. The only situation where tax is not applied is when someone trades BTC for another alternative medium of exchange, such as LTC. Because there is no laws governing the trades of "virtual currencies", that could be considered a barter. However once the trades involves any another good tax will be applied if that results in profit.
Ask your self: if BTC is so good for tax evasion, why are not Google using it? It is well know that companies like Google do whatever they can to evade tax. So why are not Google accepting BTC?