1 person using encryption vs 1 million people using encryption.

Sure, they won't go out and arrest all one million, but it's basically security by obscurity.

Once that's the case, otherwise legal activities (e.g. protesting, or making political statements) run the risk of making you a target. Law enforcement can then punish you for your legal activity by selectively enforcing this other law.

The resulting situation is one where everyone knows to some extent "you better shut up if you know what's good for you", and puts a chilling effect on otherwise legal forms of civic engagement.