Cariou/Prince case is one of the most notable ones of the recent decades about the violation of the art copyright law. It was an interesting read. I understand Cariou but I think the biggest problem he has is that Prince sold the works with his appropriated images for a lot of money and he's got nothing apart from the undoubtedly increased popularity.
I think the reason for not asking the author for permission may lay not (or not only) in the arrogance and ethical ignorance of the appropriator but in the nature of the appropriation practice itself - the provocation and controversy seems to be its important if not crucial aspect.
I imagined what would happen if Price contacted Cariou and ask for the permission. Cariou would probably refuse or come up with some rules of do’s and don’t’s for Prince which would probably totally changed Princes's perception of the images and affected his ideas.
I think the aspect of not knowing about the author or not wanting to know is a critical to achieve whatever sparkled at the first sight. There are many artists who appropriate others works but as far as I know these are very often anonymous images (John Stazeker, Tom Butler)