Individuals often think society owes them everything and that they owe society nothing.
Richard Prince took some Instagram images created by others and made money on them, and he did something similar with other photos. Whether his use of these images was legal or ethical depends, for me, not on the single value of the creators’ rights, but on the dual values of the creators’ rights in light of the greater good.
Prince's work is not about public domain. It's basically about violating copyright and/or fair use.
As far as I know, not a single piece Prince has appropriated is in the public domain.
Not one. If he was exploring the concept of "influence," he would draw as much -- if not more -- from public domain than from copyrighted works.
The Instagram images are not in public domain. The posters retain their copyrights, even if they do not have the resources to defend their copyrights in court. Prince did not ask permission to print and sell their works, and barely transformed the works, thus there should be little doubt he violated their copyrights.
As noted, the only possible legal and ethical rationale he might have for that series is a fair use claim. I don't think the minimal transformation in that case in any way justifies a fair use claim.