The state literally prints currency to facilitate commerce, so it is beyond unjust to then seize it from people who use it to conduct commerce regardless of whether there exist other means of doing so.
I sold a car to someone on craigslist for a few thousand dollars. The person who bought it paid in cash, still in a bank envelope. This transaction was not criminal, and it's exceedingly unlikely that the money had illicit origins. I promptly deposited the cash into a checking account. The amount certainly prompted a Suspicious Activity Report to FinCEN, which I made no attempt to avoid, as doing so would have indeed been criminal.
Cash is just by far the most trustworthy form of payment to accept from strangers (other than perhaps cryptocurrency or physically going to the bank with them), which is why craigslist warns against money orders or checks of any kind.
It would have been entirely and unambiguously unjust if the state had seized that money from either me or the person I legally sold my car to.