In vast majority of countries even possession of drugs is illegal. Drugs trafficking and owning large quantities is usually illegal even in countries where possession for personal use is legal.
Weapons are even worse because of how liberal USA gun laws are compared to the rest of the world. There are many countries where the process to get a gun is much harder and full of bureaucracy and requires registering with the police.
Do you seriously want to make an argument that all of these guns and drugs are all Portuguese and Czechs buying 1g of coke or 10g of weed at a time, Swiss and Americans buying guns that are legal in their countries, registered with the police, etc.?
Don't be ridiculous. Tor and Bitcoin are used by criminals and it's not a coincidence.
And then there is the morality argument. Bitmixer owners felt their service was used by criminals (their words) and it left such a bad taste in their mouth that they shut it down on their own, there was no legal system involved. I can get behind that choice if that is really their reason. It's not like being an asshole is illegal but many (most) people still hate assholes and won't help them in any way. Just because something is legal (and again - drugs and weapons usually aren't) doesn't mean it should be done, applauded and enabled by convenient websites.
If Internet users have the power to make their own law, their definition of criminality will be as "reasonable" as any other.
Edit: Amish users have the right to forgo Internet use. US/EU users have the right to forgo drugs and porn illegal under US/EU law. Same for Saudi users and Saudi law. But no one government has the right to rule the Internet.