People have the right to utilize their copyrights however they want, and that includes releasing it into the public domain, or under creative commons. But if they want to control the copyright a little tighter and release it only to people who pay them, like in your commission example, that is also their right, and not yours to just take it.
Pirates are not advocating we "just take" private information. If the commissioner for some reason now chooses to make his property publicly visible, that doesn't include anyone else in the original agreement.