I understand that you disagree, and I understand why, so let me explain this from a different angle:
Would it possible for you to stand in front of the artwork by Matisse and have the same experience that you have when you stand in front of the artwork by Thomas?
There are numerous differences between the two pieces, obvious differences being that one is a painting and one is a paper collage (these are fundamentally different media, as far apart as acoustic versus electric guitar), but also that they are different colors. However, I submit to you that you would not have the same experience looking at the two, if for no other reason than that they were created by two different people.
Regardless, everything I’ve read indicates the following:
“Thomas’s 1963 painting, Watusi (Hard Edge) was originally created as a deliberate reworking of Matisse’s large 1953 cutout collage,l’Escargot, and that it had always been recognized and discussed as such by the people who followed Thomas’s work.”
Given that Thomas did openly acknowledge the relationship (as cited in the thread above) and that she made significant changes between hers and Matisse’s work (she changed the orientation, color, and media) she has met the criteria you’re concerned about. The real questions you should be asking:
Why would a black female American painter make a painting that so obviously refers to one of the most famous white male European painters’ works? And beyond that, what does it say about us today that we’re so suspicious of such a person, even though she was forthright about the inspiration, and we know she was an accomplished and self-conscious artist, who studied at both Harvard and Columbia?
Artists like Picasso originally became famous for taking inspiration from ‘primitive’ African art and translating that style into European oil paintings. When Thomas made this oil painting in the 1960’s, she was under heavy pressure to paint in a way that spoke of her identity as a black woman, which typically meant figuratively, and yet she chose to make this painting, directly inspired by European abstraction and modernism. I think this painting is a direct question from Thomas, and she’s asking if she’s allowed to make this painting. I know you’ve made up your mind, but I humbly suggest that this painting is about a dilemma Matisse could never have made a painting about, and I request that you reconsider your evaluation of it.
Would it possible for you to stand in front of the artwork by Matisse and have the same experience that you have when you stand in front of the artwork by Thomas?
This one does not disparage the original work, but it surely exploits it.
It would seem by your unconventional legal standards, it would be permissible for any film-maker to make a movie out of a best-seller, without contacting with the author for the movie rights, no?
As far as whether the copying was revealed, it’s unclear whether that was done by apologists after the fact, or openly at the time. That would affect my moral judgment of the copying, but not the legal judgment. The advance confessing of infringement is no defense.
If you’d like to show me Picasso’s “copy” of an African artwork, I’ll be happy to apply the same standard (sort of - as copying an unattributable primitive work is different from infringing the legal rights of a copyright owner).