I think some exchanges have already charged the cost of buying and selling coins and I think that fee includes the tax paid for the development of the exchange, but for taxation the ownership of the cryptocurrency assets that you have does not have to exist.
Do you even know what capital gains tax is? The fee those exchanges take are for developing their exchange, their profit and paying their "own" tax (not yours!) if applicable. You will still need to file your own tax if you hold crypto currencies and make profit from it. The rules are bit confusing, but as far as i know, you will have to pay tax on your capital gains, at least where I am living.
It's a little complicated if you do not study these terminology, the simple term that can easily be understood is that whenever we make income, we are required to report it and pay a corresponding tax.
In my country I don't need to pay for this since there is no law yet that tackles about this matter, but when I opened a new bank account, I was ask about my source of income, and since I don't have a regular job, I just said it's coming from trading because
I am really trading but focuses on long term trade, so if there's gonna be a specific law about it, I cannot deny anymore because of the information I have provided, but until that law has not pass yet, I should enjoy my profit in crypto with tax free.