Parking fines aren't debts, and thus, legal tender doesn't apply.

https://www.findlaw.com/legalblogs/seventh-circuit/city-sanc...

> The Seventh Circuit Court of Appeals ruled this week that city-levied fines are not debts under the FDCPA... District courts, for what it's worth, uniformly agree that a fine does not stem from a consensual transaction, and thus is not a debt under the FDCPA.

> a fine does not stem from a consensual transaction, and thus is not a debt under the FDCPA.

Which transactions with the government is "consensual" where it doesn't demand payment up front (like a contractor)?

This goes back to my idea that while legal tender is a nice idea, in practice it means nothing

> Legal tender has never been challenged in court to my knowledge

It was challenged and upheld, both as against debts before the the legal tender acts were passed and those after, by the Supreme Court in Knox v. Lee (1871).

Yet the US government can refuse payments in cash

https://www.federalregister.gov/documents/2017/10/03/2017-21...

Yes. Because a patent filing fee is not a debt.

(And because government can exempt itself from virtually anything not forbidden by the Constitution. This is why cops can break down your door, but I can't.)

In the UK the definition of legal tender includes a limit on the use of small denominations.