I do not think that many traders pay taxes on their profits. Firstly it is very difficult to track down and in General how they will know how much the trader really earned money this year. Fuck them.
I would suggest it depend on the exchange and which country it is located in. I also assume that in the US at exchanges like Kraken, Polo, Bittrex, Coinbase, and their likes all US residents have to pay their income taxes whether they want or not as these exchanges directly report all transactions to the IRS. In other countries the situation with the income tax may be different, and in some of them bitcoin and cryptocurrencies may not be taxable at, which effectively means you officially might be exempt from paying an income tax on your bitcoin profits. These are known as tax havens I guess.