But that applies to anyone who owns a copyright. If you don't attempt to enforce it, you can't then sue if someone does significantly violate it. Not a Disney thing; its a copyright thing.

You're thinking of trademark law where you have to be seen to protect your trademark. Copyright law works a little differently as you can pick and choose your fights to some degree (and many artists do operate this way by turning a blind eye to YouTube bootlegs but upholding their claim when their intellectual property is used commercially without permission).