You confuse an object with wide range of utility and a limited set of nefarious uses, a knife, with a system of technology and techniques with a limited range of utility and vast capability for misuse (mass surveillance). I suspect the annual proportion of illegitimate knife use to legitimate knife use is so low, it would look stupid to even write it out.
If we round up substantially, we get about 2000 knife murders per year in the US. http://www.economicpolicyjourn... There are roughly 300,000,000 people. Let's say each person uses a knife on average once per day (spreading butter, chopping veggies, cutting string, killing people). That's 109,500,000,000 knife uses per year. 2k/109,500,000k -- that works out to a proportion of 0.00000001826484 evil knife uses per legitimate knife use.
Note: there are more than 300m people in the US, there are actually fewer than 2000 knife murders per year, and most people probably use a knife more than once per day. There are of course other illegitimate knife uses than murder, but considering that the number up there is extremely generous to your argument, we could probably call it a wash.