Google 3D Warehouse copyright issues

[Kurt M. Weber]

I can take a copy of Windows 2012 R2 and modify it by adding a script. That doesn't mean that I own the copyright for the original work.

You don't need to own the copyright to legitimately modify or redistribute something, if the copyright holder gives you permission to do so.

By uploading Sketchup models to 3D warehouse, the copyright holder has given you permission to both modify and redistribute.

Of course, while you indisputably and unambiguously have the right to redistribute and modify however you choose (subject only to a few narrow restrictions) with content from 3DW (since copyright holders are required to grant you such permission as a condition of uploading their content to 3DW), it goes without saying that whether you're required to or not, not crediting the original creator is probably a majorly dickish move.