This is why.
In December 2008, a conviction was upheld in NSW Supreme Court, in which a man was brought to trial for cartoons based on the child characters of The Simpsons, showing them engaged in sexual acts. The fact that “no real children” were depicted or harmed–often the argument used by fans in defense of shota–was irrelevant, according to the judge in the Supreme Court appeal.
“In my view, the magistrate was correct in determining that, in respect of both the Commonwealth and the NSW offences, the word ‘person’ included fictional or imaginary characters … the mere fact that the figure depicted departed from a realistic representation in some respects of a human being did not mean that such a figure was not a ‘person’.”
The LiveJournal community sf_drama may be wanking over the subject currently, but that doesn’t change what the law is for Australian fans. And it’s probably something all fans should keep in mind: make sure you know the laws regarding child pornography and how it is specifically defined in your country, or another country if you are sharing images, stories, etc with fans worldwide. You may be putting yourself or others at legal risk and not realize it.