Consider the possibility that until cryptocurrencies are legal tender, they are really just securities, capital as the guy from the Netherlands suggests.
If that's the case, and I've seen other articles here in Steemit to support this idea, then Steem Dollars and Steem and SP are really just a form of capital and that you pay tax on the gain, nothing more.
This is definitely something I will be exploring in later parts. However, capital gains only applies if you hold the asset for over 1 year. Under 1 year is considered trading income and taxed like normal income. (In the US)
I found what I think to be a relevant article, citing the IRS as a competent authority, and a ruling issued by the same. Bitcoins are considered "intangible property", so it received capital gain or loss treatment
https://www.forbes.com/sites/greatspeculations/2017/02/21/if-you-traded-bitcoin-you-should-report-capital-gains-to-the-irs/#19d5013ae3d8
I hope you find this of interest and to be useful.
Thanks.