Bitcoin can't be taxed for sure and with some tumbling tracking down bitcoin transaction is almost impossible.
In most well developed countries Bitcoin is already being taxed since the difference with money is zero. If you have $50,000 in cash, Gold or Bitcoin, it doesn't matter since it all adds up to your total wealth. However, you have the obligation to declare all your Bitcoin holdings and the profits that you have made in order to come into a position of actually paying tax. Your honesty plays the most important role here.
If you declare everything honestly, then you pay tax. If you keep your stash away from the banks and governments and don't declare anything, there is no way they can see what you have, and thus you can avoid having to pay tax over what you have in Bitcoin. If you put the option of declaring your holdings and not declaring your holdings, then the easiest and cheapest way is to keep your mouth shut. That's why Bitcoin is seen as a great escape by a lot people here.