In fairness, Disney would be able to do this whether the character remained sequestered or not; nobody else can, though- what a shame.
The fact is that if you're not making money off these characters, you can do whatever the hell you want. If you are making money and you can keep it fairly anonymous, you can still do whatever you want.
I find it kind of offensive how Disney has been putting Steamboat Willie on its opening credits, recently seen in Zootopia, flaunting its perpetual copyright in our faces.