can you point me that rules? because i know that bitcoin itself are not taxable, but only their conversion in fiat
http://www.irs.gov/uac/Newsroom/IRS-Virtual-Currency-GuidanceBitcoin itself isn't taxable, no, but exchanging for fiat isn't the only taxable event. Bartering is also a tax event, which includes spending or trading altcoins, for example.
i can use bitcoin to buying used thing or food, and they won't track back something like this it's purely impossible...
The US tax system is an honor system. If you generate gains from spending Bitcoins that have increased in value, you are supposed to declare it. Poor recordkeeping is not an excuse if you were audited.
how so? something like this could only help adoption(as long as everyone will do it, merchants too), because it prevent dumping btc for fiat
It's bad advice because it's wrong.
But yes, the advice goes against published IRS guidance.