They are founders who put money in, shares were created, they were given X% of shares.  They may also gain shares by agreement from those held in reserve over time.  As owners shares are not income, just ownership in a company YOU INVESTED IN. 

The only way you pay tax on stock like that is to cash some out.  Then it is subject to capital gains.  Say tomorrow you buy 2 btc for 50K each worth 100K total.   Price goes to 100K each by the end of the year.  You now have 200K USD in bitcoin, how much tax do you owe?

The answer is zero unless you sell it, convert it or pay for something with it.   It is no different.   The graph really has been over simplified, but it's a meme soooooo