It's not difficult to know who had been trading in cryptocurrencies but it difficult to know the amount traded and to whom and what was it exchanged for.
Well, places you buy and sell crypto-currencies are subject to financial reporting regulations. So, the IRS can find out how many you bought and sold, how many you transferred, and how much you hold. Then, if you get audited, you have to show the IRS that the taxes you paid can account for all of that.
And how are they supposed to tax an extremely volatile currency?
If you read the IRS reporting rules, they're taxed just like any other asset, such as stocks, bonds, precious metals, etc. What you pay taxes on is the change in value between when you bought and sold. You should likewise be able to write down losses, if you sold for less than you bought at. If you make a financial transaction other than for $USD, I guess the spot price is taken as the value, for tax purposes.
today (ex: before the tax filing) 1 bitcoin might be 20Grand USD but tomorrow (ex: the day after submitting the tax form) it may be 5Grand, so how much should they pay for tax?
You're not taxed on holdings. Only sales (which includes exchanging crypto currency for goods or services).
What the IRS is doing now is just fearmongering.
You
really want to call their bluff? If you get audited, you're screwed. Before you decide to fight it, maybe ask Wesley Snipe how that worked out for him.
https://en.wikipedia.org/wiki/Wesley_Snipes#Income_tax_conviction
Maybe you can fight the IRS if you're a billionaire. But, if you're a mere mortal, the IRS pretty much always wins.