You are comparing harmless messages to dangerous weapon. It would be ridiculous if one could buy a rifle with a range of hundreds of meters but couldn't use an algorithm to encrypt a message.
Who says messages are harmless? I disagree vehemently and contend that information can be indeed more harmful than firearms in the hands of the right people.
A single message can mobilize millions of people to take to the streets and riot. A single firearm can't do that.
A message sent to millions of people will be either public or will be leaked. So it doesn't make sense to ban encryption for such messages. Better ban riots. Also, there is something wrong in the country if millions of people are ready to riot.
