I think the reason why they tried to prosecute that is because they use the gateway principle. That it will lead to normalising images of, mainly, young girls being sexualized, and that will lead to more real life settings as that is generally how porn works for many people.
The gateway theory is part of the reason real images are banned, as well as it encouraging a market for its production.
I think fictional images are illegal in some countries.
There's absolutely no evidence that the gateway theory holds any water. It's often repeated, but it's not proven.
Personally, I don't find any link between fantasy and reality. What happens in our imaginations is often totally beyond the realms of anything we'd act out in real life. Who hasn't had a daydream about killing someone? Honestly? I think we've probably all daydreamed about killing someone. Perhaps brutally. Holy crap, we should all be in jail for this thought crime? "It's a slippery slope..."
For the sake of argument, there are some (with whom I don't necessarily agree, btw) who say that such cartoons might prevent some paedos from attacking real children.
Most anime characters all look underage, and that seems to be acceptable?
Nope, in the UK they're technically illegal. The criteria is that a "reasonable person" could think they were under 18.
Take your average catgirl. In most fantasy universes, they age like cats and not humans. So an 18 y.o. catgirl is geriatric
The whole thing gets more absurd, because the law as it stands does not limit "being a child" to being a) real or b) human.
It is always illegal if a "reasonable person" takes a look at it and says, "they're probably under 181". A drawing of sentient anthropomorphised plant could make you a sex offender
e: 18, 81, same difference