I think the reasoning is fine, because of these words: "...if the behaviour which is currently criminal is to remain criminal..."
Your example is a simple crime, where the victim had an experience related to the crime (so there's a body to be found by the police, or a surviving victim who says "ouch, someone shot me"). They are talking about certain types of crimes where neither the victim nor anyone closely watching the victim would never have any idea that a crime happened. All the evidence is completely disconnected from the victim.
I publish a magnet link. You read it, and use it to acquire a file. Someone who isn't there and sees absolutely no effect on their life, is defined as a victim because the action is "currently criminal." Maybe it's because they hold a copyright on the contents of the file, or because the file contains a picture of them without clothes (taken by hidden camera when they were 17 years and 364 days old), or because the file contains some other information related to them.
You can't detect these kinds of things.
The House of Lords is saying that if these are going to remain crimes, then the laws should be enforced, and if we ass/u/me that getting laws enforced is far more valuable to our society than liberty, efficiency, etc then it's important that the watchers know about every transaction that is happening and who is involved. They need to know that I transmitted information to you (and who both of us are) and what that information was. Until they have all that information, they can't even begin to guess whether or not a crime occurred. Maybe the file contained a picture of my dog rather than a 17-year-old human, and they need to know who took the dog picture and that I sent it to you, so that they know it wasn't a copyright violation.
Of course it's absurd, but that's because the premise is absurd. Their reaction to it, is quite rational. But that's my point: it almost looks like (especially in the paragraph that I quoted) they might be calling the bluff, pointing out the inevitable consequences of having externally un-detectable things be crimes. If they weren't that clever and didn't mean to do that, too bad, but even if it's an accident, they did it.
It's not an accident, though. Look at it (emphasis mine): "if it's to remain criminal" (see the wiggle room there?) and "currently criminal" and "there is little point in [doing this] at the same time [as doing that]" and "difficult question."
I'm not saying this is ingenious, but it really is a fairly well-crafted.