How can they require taxes for those you uses Bitcoin if they do not know who uses them. I bet if they are going to do that then they might give the exchanges to pay taxes for us then the exchange would get the taxes from us.
Taxation, regardless of Bitcoins being involved or not, is pretty much an honor system anyway. You can get away with under-declarations if you're confident the systems in place won't detect any wrong-doings. In this particular case, exchanges are going to be (or already are, in some countries) integrated into that system by monitoring them. That means if you use an exchange, and not declare your Bitcoin income, you could very well be charged for tax-related offenses.
If you only engage in peer-to-peer trading, you're right, they won't know you use it for the most part.