If your BTC are in a personal wallet, and a hard fork/airdrop occurs.. If you are unaware of it, and never download the new wallet... It will prolly come down to how understanding the IRS auditor is.. either you owe it, cuz you *could* get control if you downloaded the new wallet, or you don't owe, cuz you obviously wouldn't download a wallet for something you were completely unaware of...
I think this makes sense. What if there was some Bitcoin Gold, Bitcoin Green, Bitcoin Purple, Bitcoin Silver, Bitcoin Platinum, Bitcoin Diamond, Bitcoin Whatever ...
And you never downloaded the wallet, so the fork coins actually never moved ... when they do decide to tax you, the value of the coins at today's rates should apply, otherwise you can just say, I'll pay you in the forked coins ... What is your Bitcoin Whatever address, I'll send the tax owed on it.
That would make more sense.
But then, I don't know if the IRS will interpret it that way. They can't tax you what it was worth yesterday, when you are paying them today. As any gains or losses were unrealized.