Laws aren't always written down. A good example is what happens if you criticize Israel - the government, if it notices you, will come down on you like a sack of bricks, even though there's no law that says "don't criticize Israel". https://youtu.be/zJt3omLLAuA

Likewise, there's no law saying "don't accept payment in Monero" but you may be jailed for money laundering if the government notices you.