The "duress" part comes from the Uniform Commercial Code (UCC) which are laws governing all commercial transactions within the US. It still doesn't apply because getting a ticket isn’t a commercial transaction, but SovCivs believe everything is a commercial transaction including interaction with the government.

UCC 1-308 is a section that says a party can partially fulfill a contract without agreeing to all of the contract terms. They must do so by explicitly stating they are completing part of the contract "without prejudice" or "under duress". That's why SovCivs write these words on everything.