It shouldn't be. The original photographer of 173 (Keisuke Yamamaoto) has no say in the concept of 173 itself. His ability to copyright goes as far as the picture and design and that's it because its his picture thus his design. However, 173 itself was made by the Foundation Wiki which information retaining on there is for free use.
Let me explain.
The image of SCP-173 (officially called Untitled 2004) was given to the Wiki by Kato for the purpose of the wiki however, he never released the image by the Creative Commons and because of this he holds copyright privileges to the photo. Those who use the photo or its likeness is able to face legal actions.
However,
The text on SCP-173 wiki is different. The text of the wiki (so everything about 173) was released in the creative commons BY-SA 3.0 and because of this anyone is able to freely use the concept and information on SCP-173 Provided they do not use the image restricted.