If you browse for assets, use the no-ai filter. That is implemented as a tag filter, but all the results will have filled out the ai disclosure question with no.
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The reasoning for the disclosure are indeed legal concerns. "due to legal ambiguity around rights associated with Generative AI content".
It is one thing to have a game with ai elements, but having assets that involve ai and unknowingly use them in a project to then claim the project does not have ai would be false advertisements. There might also be platforms that ban ai content which put the creator of the new project at risk. And of course that legal ambiguity, as there is dispute about who holds the copyright, if any, for ai results, as some think there is no humans involved and only humans can hold copyright. So if the ai operator might not own the copyright, that publisher might have problems with that part of the tos: "Publishers affirm, represent, and warrant that they own or have the rights, licenses, permissions and consents necessary to publish, duplicate, and distribute the submitted content."
My opinion is, if a photographer can own copyright of a taken photo, so can an ai operator of a prompt result. But there is also the notion of a minimum level of creativity. This is more apparant with written texts. I dimly remember some companies trying to "copyright" their business emails by claiming so in the mail. This is not only bogus, the business communication just does not meet that criteria of minimum creativity in most cases to even qualify for copyright. And neither would prompting an ai to give you a pic of an apple and using the first one to sell as an asset.