Historically (As in legal historically) child pornography was a crime because it was in fact evidence of another crime (namely child prostitution etc).
Of course, it's not quite the case now, where there actually needs to be no victim for an action to be a crime.
The current ruling isn't that surprising, only because of the fact the UK has had a history of having very overreaching laws, particularly when it comes to children. There are quite a few cases where couples have been deemed unfit to keep their children because of the decision of a welfare worker in the employ of the government.
(I don't have the link to an example, although I do recall that there was a couple who were deemed too stupid to have children a few years back. Apparently having an IQ in the mid 80s is deemed stupid enough, I guess. The case ran for several years, with the council appealing every step of the way, and even attempted to prosecute one of the judges for 'ruling the wrong way'.)
You'd think it'd end there, but not really. The UK and Australia have very similar legal systems (We share the same legal background, and you even used to be able to appeal Australian Decisions in a UK court) and here's something to wrap your head around.
Australia is the only country in the world which has in its legal code 'appears to be' as part of its criteria to determine if a criminal breech has occurred.
Or to be more precise, any image of someone who appears to be under the age of 18 is considered to be child pornography. This is due to the fact that a minor cannot legally provide consent.
This causes quite a legal quandry, for a variety of reasons.
I've been called up as an expert witness over one such case (Which I am legally not allowed to identify, on threat of prejury) where I was required to give my expert opinion where a line depiction of a person who legally did not exist was over the age of consent.
I had a fun hour in the court room, and was examined and cross examined. Point of law was made that I could not say that the person did not exist, and consequently the question was invalid. I was required to actually make the determination.
I've also been called up to provide an expert opinion if an adult, (who has a passport who can certify that she was over the age of 25) appeared to look like a child, and therefore images of her were considered child pornography, due to the fact her IMAGE did not appear to be able to give consent.
In the second case, the said image was on her boyfriend's phone, and she suffered stunted growth, and the phone was inspected when she arrived into the country with said boyfriend. The image in question was NOT taken in Australia.
... That 30 minutes in the court room was downright insulting. I was not allowed to point out the fact that the woman in question was in fact legally able to provide consent, due to the fact she was, you know, 25, and quite able to make decisions for herself.
In more mundane cases, I've been in discussions with various legal people previously about defining a 'legal body' due to the previous discussion, and it actually caused significant legal issues... with the Anti-discrimination Act.
Namely, it prevented women (and some men) of certain body shapes from legally participating in the adult industry, EVEN THOUGH they were legally old enough to provide their consent.
(Or in short? You cannot discriminate employment due to age, sex, gender, martial status, disability, or you risk significant fines and jail time. Thing is, if you DON'T... well, you get arrested for the IMAGE failing the 'appears to be' test, and you just end up in jail and on the sex offenders list.)
I heard last that the UK thought the Australian law was good (somehow) and was looking to introduce said provisions to their act. I didn't follow it all the way through.
But yeah, you'd be surprised what legal fun you can have with child pornography of persons who do not exist.
(There's other countries with various laws surrounding sex and depictions thereof, but they're more based around obscenity clauses. Or basically 'We don't like showing anyone images of anyone having sex/Particular groups having sex (Singapore and Mass Effect, as an example)' and they're actually more clear cut, because they focus on the actions most of the time, not guesswork.)