It's not about the damages. It's to discourage trying out illegal commercial sharing as a business model. Kind of like the different degrees of murder - the victim is just as dead, the damage is the same, but to discourage planned illegal activity the punishments differ.
I think this actually makes sense, since deterrents likely work best against people who rationally plan their crimes and weigh the possible outcomes. A commercial pirate is likely to carefully consider the amount of potential fines. A college student, less so.
And how, exactly, do you propose to do that? Your body's own immune system can't do it, and it's had a 65 million year head start.
With cool nanomachines, of course. And the most part of that 65 million years wasn't spent on fighting cancer, but other stuff that would kill me sooner. Cancer is a relatively recent problem, I think. And even then, most of that time my immune system has probably been plotting against me, so that I wouldn't live too long and compete of resources with my offspring.
I have read the court papers on the case you refer to (as a part of a computer law course). The guy had set a program to download some newsgroups full of (legal) porn, and he discovered later that some of the pics were of children. He had deleted the ones he found and later testified that he hadn't been sure if he'd gotten them all (how could he, he can't check the age of everyone in a huge amount of pics). The court reasoned that even though they agreed the man's possession of child pornography wasn't intentional, he must have thought it possible that everything on his hard drive wasn't legal and hence he was found guilty.
As I recall, there was also a weird twist in the case where the police confiscated his hard drive to use as evidence, but after compiling a list of child porn on it they somehow managed to lose the original and all the copies, so the defense couldn't use it in court.
I have a theory that it's impossible to prove anything, but I can't prove it.