Nobody can do that. Taxing bitcoins is impossible because bitcoin is decentralized so who will get the tax? Governments? Nahhh... The exchanges and the regulators maybe they can get tax with them but to bitcoin holdlers is just impossible.
This is not quite true.
Bitcoin transactions and profit from them are already taxed in many countries. Maybe Bitcoin itself as a coin can't be taxed, or in a very hard and unefficient way, but the income that you get from it can. It's written in many existing laws in many countries. You have the duty to report Bitcoin trading income to your tax authorities.