> If the needs of the many don't outweigh the needs of the few, then why does the total number of criminals matter?

Because the victims matter.

> If I want to buy strong encryption with no backdoor, and I'm not a criminal, then why would the issue of crime matter at all?

Being a criminal isn't an attribute of a person, it's the actions the person carries out. I don't give a shit about jaywalking, but I care about rape victims. This is not inconsistent.