What's that word? Theft?

I'm so uneducated. All these years I believed Roy Lichtenstein was inspired by comics, a good artist who only later put his stuff on large canvases. Not really. Rarely he gave the original creators (Russ Heath, Tony Abruzzo, Irv Novick, Jerry Grandinetti etc) credits. What's that word I'm looking for? Theft?

For a cool collection of comparisons (where you clearly see how much more action the originals have) click the picture for David Barsalou's LICHTENSTEIN PROJECT.