Drugs are a canary in the coal mine. If you lack the ability to share drugs over a medium without surveillance, then you know that you lack the ability to share information over a medium without surveillance. If you can't say "mail me three marijuanas please" without somebody listening in, then you can't say anything without somebody listening in.

There is no such thing as a system that provides privacy only for legal activities. Therefore if you want to fight for the freedom and ability to share information without surveillance, then you must design systems that can, incidentally or otherwise, be used to share drugs (and you must fight attempts to surveil existing systems.)