It's not an assumption. It's the reality of the information age. The only way to have complete control over information is to not publish it. Once data's out there, there's virtually no way to control what will be done with it. Creators started from a lost position: they want to publish their works and yet they want to somehow control what happens to "their" data. The level of tyranny necessary to accomplish such an end requires the destruction of free computing as we know it. We're already seeing shades of it today: computers that aren't really ours, they only do some corporation or government's bidding. It's only a step away from such digital copyright enforcement nonsense to far more serious matters like cryptography regulation.
So I'm not the one assuming anything. It's creators who live under this notion that they own their creations. The truth is public domain is the default state. Intellectual property laws are responsible for bending reality and introducing this assumption that you can even own ideas to begin with. That was workable in the age of printing presses but not in the 21st century where everyone has multiple powerful globally networked computers at home. I for one think computers are a far more important innovation than almost everything humanity has ever created and I don't think enabling creators to continue living under such illusions is important enough to cripple the potential of computers. I want society to eventually reach a post scarcity state in the real world, mirroring the digital world. I don't want corporations creating artificial economies where there are none.
All creations are just data, and data is just bits, and bits are just numbers in base two. All intellectual work comes down to humanity discovering a really big unique number. How is it even sane to claim ownership over such a thing?