It’s the same rule, “fair use”. Copyright isn’t absolute, it needs to strike a balance between “give creators control of their thing” but also “people deserve to participate in our collective culture.”

Making a one-off drawing of a character and not trying to make money off of it likely checks the fair use boxes (it’s an explicitly fuzzy system, so a trial would be needed to say for sure if it’s fair use or not). Whether the training set for a generative AI system is fair use or not is still an open question, but many feel that it can’t be, as it’s operating on a massive scale (basically every image ever created by humanity) and has the potential to eliminate the entire industry of humans selling the art they create, which copyright is supposed to protect. Ghibli isn’t going to be harmed by someone drawing a picture of their characters for a meme. It could be harmed by another company making money off of mass production of knockoffs of their style which were created with thousands of unauthorized copies of their direct artwork.