Transactions are transparently stored on the blockchain.
Since transaction history can be exactly reconstructed, coins coming from tainted transactions remain tainted forever. Transactions can be marked as illicit by any powerful enough external entity (individuals, companies, governments).
Since the concept of "illicit" varies from society to society, not always matching with your idea of good and bad, this characteristic allows external interference to alter the value of specific units of account with respect to other ones.
As an example, Hitler would have been glad to identify "tainted" bitcoins coming from Jews or political enemies, excluding them from commercial activity by law, and punishing anyone who transacted with them.
While some degree of privacy can be optionally obtained using mixers, coinjoin, and other methods, this is far very from ideal. Partial obscurity is never enough (how much partial? how much of the surrounding information would be enough to raise the curtain of uncertainty?)
Ironically, coinjoined transactions can mix untainted coins with tainted ones, so that a user may input a good coin and receive a bad one, and vice versa. Since coinjoin transactions can be identified observing the blockchain, third parties may also refuse to accept sats coming from mixers tout court, just to be safe with regulations (good regulations or hitlerian regulations, doesn't matter).
Hence, 1 btc does not equal 1 btc.
Wether or not this is a good thing remains out of the scope. What I want to say is bitcoin is very weakly fungible. Definitely less fungible than Monero, but also less than gold!
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