Reminder: if you want to bet on an asset's demise (i.e. short it), you don't need a derivatives market, you just need to be able to borrow the asset and sell it. So you could accomplish the goal there by borrowing Monero and converting it to USD. A lot of smartcontract platforms let you do this -- including on other chains, where they hold a token convertible into the original chain's native unit.

I bring this up because people are always asking what platforms are allowing me to short cryptocurrencies, which seems to miss that it's enough to just have a debt denominated in what you want to bet against.

Yeah, but the moment that happens they will confiscate/block the funds of the shorters.

Based on which specific law or rule?

Based on how the crypto world works 100% of the time.