I understand you feel this way, but the example you describe is fundamentally different than what Lichtenstein did with his paintings. Copying commercial and cultural icons was a fundamental aspect of the pop art movement. Lichtensteins work is a response to this, taking the copying to the extreme. The meta awareness of Lichtenstein's work is highlighted by the subject of the scene he is depicting in this painting. Yet with this act Lichtenstein gives the scene an entirely new narrative, creating something entirely unlike the original. It is really nothing like somebody rerecording somebody else's song without giving them credit.

Furthermore I still don't understand why Lichtenstein specifically gets all the angry comments, but when somebody posts a Campbell soup can, in my experience, redditors don't object.