There are many non criminal use cases for private electronic transactions - just as there are for cash. So why is it a crime to build a thing that has both leg

There are many non criminal use cases for private electronic transactions - just as there are for cash. So why is it a crime to build a thing that has both legal and illegal applications, like knifes, guns, dollar bills, gold coins, ... ? And don't get me started with "Most activity on tornado is illegal". It is a privacy service. So unless it is broken we don't know this. Are we going to take the word of the "Authorities" for this?