To be honest, everything can be taxed even your bitcoin earnings. For example you bought bitcoins in 2013 for $300 and sold it today at $8,000. That $7,700 is taxable as income, the classification and how much it should be taxed will depend on your local laws. So you better check if how much tax you should pay. However, if you are holding bitcoins as it and profited because of the increase, unless you cash that out and sell it then there is no profit to speak off. Right now that is the only way they can tax the use of bitcoins for regular users, but it may be different for bitcoin exchanges.
That is where you are wrong because the income gained from trading or investing is not taxable under income tax but it is taxable under capital gains tax. Imagine buying a piece of land for 100,000$ and later on you sold it for 250,000$ the 150,000$ is taxable to capital gains tax. Some countries even exempt them from capital gains tax if they bought another land with that 250,000$ they earned.