This legal principle does not apply to Bitcoin, which could lead to Bitcoin's demise. This could happen tomorrow - the feds could declare that this list of bitcoins [insert list here] had been used to pay a ransom or to buy drugs or support terrorism and were now forfeit. Obviously there would be no way for the feds to actually confiscate said Bitcoins, but this would effectively keep them off the exchanges and make them worthless.
This is why I believe that one of the anonymous cryptocurrencies (of which Monero is currently the frontrunner) could eventually replace Bitcoin
There are some serious gaps in your reasoning
Let's assume that a government declares that some bitcoins are illegal (for example, in this or that wallet). Does it mean that a person who happens to hold these bitcoins can't spend them? If you think that he can't spend them, what exactly will prevent him from doing just that? Will the government be able to revoke his private key or can it make exchanges in other jurisdictions reject them? I guess that it won't and can't. It can't even prevent him from sending these bitcoins from one his wallet to another (I remember you said something about Bitcoin not being truly fungible). In this way, the government can as well ban the law of gravity tomorrow (and some American court is rumored to have actually done that), but does it mean that there will be no more gravity? Let's get real here. As Napoleon once said, and his dictum fits here perfectly, one should never forbid what one lacks the power to prevent
Correct me if I'm wrong here but I understand it's possible to track specific Bitcoins as they travel from wallet to wallet. Of course it's true that gov't couldn't prevent the use of "confiscated" Bitcoins, but as I said they could prevent their use on exchanges within their jurisdiction. If someone owes you Bitcoins, would you rather be paid in "clean" Bitcoins that you could sell on an exchange or in "confiscated" Bitcoins you couldn't sell? Remember Gresham's law (bad money drives out good). People would hoard their "clean" Bitcoins and try to get rid of their "confiscated" Bitcoins. At the very least this would create two classes of Bitcoins
I'm inclined to think that you are incorrect in your premises
You say that Bitcoin is less fungible than cash while it is certainly the other way round (if we are not talking about nickels, of course). Every paper bill has a serial number on it, so, strictly speaking, it is unique and can be distinguished by this number from other such notes. That's basically why they have serial numbers, to easily tell one bill from another. In respect to Bitcoin, after you send, say, 1 BTC to your wallet which has a balance of 9 BTC, you won't be able to tell which one among 10 bitcoins is the bitcoin that you sent to your wallet (Bitcoin address). In this way, it is easy to understand that Bitcoin is in fact by far more fungible than any paper money out there. So what you suggest is simply impossible to implement since ultimately you would have to mark as dirty all existing bitcoins and the government will have to ban the entire Bitcoin network
Suppose that it were a felony to trade "confiscated" Bitcoins. Are you really so confident that you can cover your tracks that you would be willing to risk prison time to continue to use your "confiscated" Bitcoins? I wouldn't. Would you continue to buy or accept even "clean" Bitcoins knowing that they might be blacklisted at any time? I wouldn't do that either
Only certain transactions can be marked as dirty, not bitcoins which they are used to transfer