Just because Disney can stop you from commercializing something does not mean they own the copyright of the entire work.
Suppose you make a Yoda painting. You do not own the rights of _Yoda_, but Disney does not own rights of your painting, either. If you make a painting of Yoda, Disney can't commercialize and sell it either.
In this case, Disney owns some rights, and you the artist have rights. But it doesn't mean Disney "owns the copyright" to your work. They cannot publish/commercialize your exact Yoda painting and put it on t-shirts to sell.
Suppose you make a Yoda painting. You do not own the rights of _Yoda_, but Disney does not own rights of your painting, either. If you make a painting of Yoda, Disney can't commercialize and sell it either.
In this case, Disney owns some rights, and you the artist have rights. But it doesn't mean Disney "owns the copyright" to your work. They cannot publish/commercialize your exact Yoda painting and put it on t-shirts to sell.