Umm phoenixamon, err..., you saw the target and missed it by a mile. ;)
CL doesn't claim ownership of third-party content. What they do is claim copyright for the format in which this content is stored.
I can't reuse a light set for Poser in Cinema 4D. To do this I or someone else would have to write a C4D plugin which can read .pz3 or .lt files. This means someone has to reverse engineer the file format, which is not allowed (see clause G. of the P5 EULA).
In effect all those small utilities which work with Poser file formats (MorphManager, PropViewer, CR2Editor, ...) would be illegal or need a written permission from CL.

I don't believe this is what CL wants, but if you follow the letter of the EULA it is what you get. This is not CuriousLabs fault, every EULA of every software I read is to ambiguous. CL is just in a position where I can bitch at them. Sorry guys. ;)