The one that is missing the nuances is you.
1)Plagiarism and IP laws are related but different concepts,
you can incur in plagiarism without infringing IP law, and you can infringe IP law without incurring in plagiarism.
2)Even in the sufficiently transformed artwork that you mention credit to the transformed work is due so that curators can see what exactly the transformation consists of, what are the additions and what belongs to the transformed work, for in its absence the whole work is assumed to be the work of the user, which is not the case.
3)It's not precisely the same legal principle or rationale that allows for transformations in the form of parody, which is based on the fact that mostly nobody would authorize a parody of their work, even though both fall under the broad category of fair use, they are distinct principles and rationales.
4)Even in the case of images in the public domain or that are free from copyright, which legally can be used in whole or part without credit or references, credit is due on Hive nonetheless since in it absence the image is implicitly assumed to be the work of the user, especially when posted in artistic tags, and hence rewarded monetarily under that assumption. Just like you cannot mount a commercial, photography exhibition with pixabay photos claiming to be the author of the photographs or publish a written work that is in the public domain and claim you are the author, for in such cases, even if you are not infringing the license or distribution rights, you are incurring in plagiarism, which is taking the work of others and attempting to pass them as your own.
Thank you.