It's about like putting a fake speed sign up that says do 80 MPH and then giving you a ticket for doing it.
If an adult lures a 13yo for the purposes of sex, then he or she should have the book thrown at them hard. However, if the person involved isn't actually 13 (or a minor) then the court is correct...the elements of the crime weren't all there since what makes the act illegal is the age of the participant.