This principle guarantees that at the very least one will not be criminally persecuted for acquiring, holding or selling crypto assets if there was no law criminalising these actions at the time when they were executed.
Yes, that's right. As far as I know, one is deemed legal when there is no law declaring that it is illegal that's why using cryptocurrency in a country when there is no rule of banning it means that people are free to use cryptocurrency and spend it to whatever they wanted as long as what they are doing is allowed by law. When they spend it on something that is illegal like buying drugs or use it in criminal activities that would render it as illegal.