It's the government trying to enforce their opinion of who should own those Bitcoins, thereby taking power away from the owner that the network has decided on, which would be "whoever has the cryptographic keys".
9/10ths of law is property ownership. I find hilarious that anyone would want less of this concept, not more. My interpretation is that crypto enthusiasts want the "trust of the crowds" not a centralized government. Which doesn't mean the trust system becomes contracted per se, but rather under a different set of rules (i.e. purely direct democracy vs centralized republic).
Another way of putting it is this. Expansions in the legal concept of property are consequently expansions in the dominion of the state.
So your concerns are about restriction of liberties? or society moving away from small government?
Are you a supporter of cryptocurrencies?
When the government steps in, it's not usually to say _you_ own it but rather _they_ own part of it. If you own it of course you owe them nothing of it. Government finally getting their greedy hands out of private property would be a huge step up.
This is even more obvious when I present it this way. Yesterday there's a cow in my yard. The next day my neighbor says he recognizes the cow as my cow. The neighbor has ceded over power of the cow, now recognizing I have full control of the cow and they're removing any claim of power they had. It's a contraction or break even of, not expansion, of my neighbor's power.
Ownership means you have full control over your crypto. Paying taxes on it means you don't have ownership, but rather partial ownership. If the state is conferring you full ownership, it would appear their power is decreasing. If the state is taxing crypto, then they're taking away ownership status and instead attributing some of it to themselves -- THEN it's an expansion of power.