Copyright law is still in a "funny grey area", especially when it comes to fan art.
Most Copyright holders are fine supporting fan works, even the sale of them, because it helps promote things. Other times, especially in the case of Marvell or Nintendo, they are more strict on what you cannot do.
Copyright law is still in a "funny grey area", especially when it comes to fan art. Most Copyright
Also humor helps, copyright is complex but when you involve a joke you can make an argument for parody, which is protected. But the only way to know if something is legal or not is to take it to court, which most companies don't want to deal with when it comes to fanarts. The few examples didn't go the way they could have wanted, and the more examples like that the worse for them.
Also humor helps, copyright is complex but when you involve a joke you can make an argument for paro