This is true in principle, but what makes Bitcoin's case better is that the mnemonics in question are small, and can actually be remembered and not have to be committed to any medium at all.

Child Porn, for example, is also just numbers - but not that easy to remember in any reasonable representation. So, it has to be committed to a medium, and reasonable people might make laws that says that if these set of numbers are interpreted using this parser, and the output looks like this image to the eye - the person who wrote the original numbers can be arrested, or some such.