And the feature of many monetary systems is they are "property" i.e physical currency, and so must yield to US legal doctrines about physical property.

It's not relevant that this is US-specific, because this is an example, which must be based somewhere.

> any kind of property.

I'd imagine that includes BTC too

I don't think it does. Rights to search & seize physical goods isn't the same to search a digital device and force a transaction. At the very least, you can with-hold passwords i.e. refuse to participate, in a way you can't prevent police from searching your vehicle.