Dana_Jorgensen said:
Okay. Store surveillance tape shows a guy walking in, stashing a radio under his jacket, and walking out. He comes back in and does the exact same thing four more times. Nobody notices at first, but he gets caught on the fifth try. Security discovers the first four instances while reviewing security footage while waiting for the cops. Cops arrive, search the guy's car, but can't find the other four radios. How many counts of shoplifting does he get charged with when the cops drag him off to the station? 5 counts, because their is circumstantial video evidence, even if they could only find one of the five radios. Proof of the download now usually works the same way as that surveillance video, thanks to a lot of recent kiddie porn cases, where "someone else must have downloaded it on my computer" didn't work as an excuse.
Bullcrap.
The case of someone downloading a PDF and having their IP discovered is much more akin to this:
A lot of gas stations have outside security cameras aligned to film the bumpers of cars that go through. Let's suppose that someone drives through, fills up, and drives off without paying. Because the camera was fixed on the bumper of the car, there is no record of the person himself (this is true of internet access: we see the equipment used to perform the crime, not the person behind it). Now, we can get some information from this film; certainly we at least know the color and make of the car. This is similar to knowing the IP address of the the perpetrator.
Knowing the make and color is information, but it is not enough to pin it to a specific person. Even in the case of someone with a non-changing IP, it is at best tedious tracking that IP back to a person: lots of red-tape and paperwork getting the ISP to release their record. In the worst case, that of a constantly changing IP, it is well-nigh impossible. Just as with the gas video that only shows the make and color, it is conceivable that we might find the person, but not likely and certainly not easy.
Now, we HOPE that the gas video also caught the license number of the car. This would be of great help to us, but it is still not conclusive evidence: while we can discover exactly which car it was, we cannot say for certain who was driving it. This similar to a case where the offender happens to have a non-changing IP and the ISP has been cooperative in revealing it. Again, it is extremely difficult to prove who was operating the computer at that instant, however.
There might be three people who drive a given car on a daily basis, any of whom could have been the gas thief. Equally, there could be three (or oftentimes more) people who use a given computer on a daily basis. Without additional evidence (like another camera that caught the actual person on tape), the prosecutor is at best looking at a tough legal battle and at worst looking at the impossible.
Having an IP record is circumstantial evidence. If the offender happens to have non-changing address (which you won't know until you've gotten through the ISP's red-tape, which isn't a picnic in and of itself), then you have strong circumstantial evidence. But it remains just that: circumstantial evidence, and one cannot be convicted on such. Hard evidence, such as a copy of the file on the offender's computer, is required for a conviction, or forensic evidence that the file was at one time on their computer.
To further illustrate the absurdity of your example, consider this:
A man steals four radios from a store, as you described before. A security officer notices it later, and reviews all the tapes. He notices that the thief is male, always wears blue jeans and a red hat, and has a beard.
Does this mean that the next bearded man wearing blue jeans and a red hat that enters the store can be charged with four counts of shoplifting? NO. A security video, just like an IP, is not a conclusive identification.
You have evidence that an unknown person with a given description (Used a Computer that had IP 43.21.354.12 at 12:35:02PM or Male/Beard/Blue Jeans/Red Hat). This does not, however, give grounds to convict some random person who happens to fit that description.