Because if one uses that logic, then one is liable to say something like, "I have some drugs that I use from when I did drugs, since people will use drugs anyway, I might as well sell them." - I think most would say that's no good justification.

The point is that if the instruments are wrong to use, I don't want to be at fault for aiding another to use them. If they get them from another source and I couldn't influence them, then its no sin of mine.