If someone is prosecuted and assumed guilty of a crime, then it proves that the law does not prevent the activity 100%. However, it also seems virtually certain that it prevents some of the crime.

If someone claimed that because people are arrested for murder all the time, the law(s) against murder have no effect, is that really serious? Or is it just the sort of thing people like to say, but everyone knows better?

If you make a statement and then don't support it, it creates the unfortunate appearance that you're not expressing sincere opinions, or if you are, that you won't explain where they come from.