Copyright is ridiculously hard. Beyond difficult.
Even if you have general guidance across the varying nations, most of the time you can't expect it to be compatible. In fact, I've run across differing and incompatible copyrights across states within the same nation. I've seen copyright laws that contradict themselves, and the courts reinterpreting that law in seemingly random ways, so understanding the law isn't a guarantee that you can obey it.
As things currently stand even determining who holds a copyright may not always be possible, and the person who holds the copyright may not be aware that they do.
Even just the concept of Public Domain does not exist in all countries, and does not mean the same thing across most countries - there are subtle differences that can lead to insane incompatibilities.
Distribution rights, whilst complicated, are far less complex than the tangled web that is copyright law.