Or arresting Ron Rivest, Adi Shamir, or Leonard Adleman for helping criminals hide their communications from law enforcement.

Communication providers are not required to obtain licenses and mitigate illegal content in the same way that money service businesses are required to mitigate money laundering. Some people think they should be! I don't agree. But it is a thing governments could do.

In my opinion, this (very popular!) line of reasoning is missing the forest for the trees.

A knife is widely regarded as a tool that's legal to own and that, save a few precautions, you can carry with you. Switchblades, despite being also knives, are widely considered dangerous and their sale and possession is overall restricted (presumably) due to a strong correlation between the presence of a switchblade and stabbings.

Just because SMTP and Tornado are protocols it doesn't mean they are the same - one is a protocol enabling a wide range of activities (some of them criminal, some not) while the other is literally designed to hide the origin of money, an activity that most governments frown upon.