The Lichtenstein pieces that you noted are part of an art movement from the late 1950s called "Pop Art." Like Dadaism before it, Pop Art had a heavy emphasis on "found art" or "found objects," the utilization of everyday objects or images that are broadly unaltered, but physically manipulated or transported to new settings in ways that imbue them with different or additional meaning to the viewer. (Andy Warhol is the go-to example of this movement, if that helps to contextualize it.) Much of the controversy around the value of late Modern and Post-Modern art centers on extreme disagreements and controversy about whether or not this kind of work is legitimate "art"...which is exactly the kind of attention and critical debate that Pop Artists and their intellectual descendants sought to elicit.

So, to Lichtenstein: his focus in the works you cited was comics, from which he took particular frames out of the context of their original story and recreated them, usually with small, but important, changes to the image, and with substantial changes in vividness and scale. While he's best known for these comics-derived works, it's useful to understand that he didn't work exclusively in this style. He produced a large body of ceramic sculptures, abstract paintings, and art deco works that were popular and respected by his contemporaries as well. The comics-based pop art that he produced is limited only to a few years in the early 1960s.

Lichtenstein was never sued for copyright infringement for any of this work, nor did anyone seek financial damages. Such a lawsuit would probably fail given the modifications that Lichtenstein made to the original comic art, although some of those original artists remain(ed) quite bitter about the wealth and fame that Lichtenstein attained. An additional complexity is that the comic publishers at the time rarely credited their own artists, which raises questions about how much responsibility Lichtenstein had to seek out the artist when the original medium was uncredited. This work certainly faced controversy and is still actively debated to this day, as you noted, but controversy in art usually isn't a liability. The only consequences that Lichtenstein experienced were positives for his career, which was celebrated and lucrative.

So, is this kind of appropriation OK? The answer depends largely on how you feel about the idea of found art. The manufacturer of the urinal that Marcel Duchamp turned into Fountain certainly didn't get any credit either, but Lichtenstein is more divisive because his found objects were already works of art, albeit of a less-respected variety in comics, which subjectively feels a bit different when "found" and manipulated. I personally tend to agree with Alistair Sooke, a British art critic who has strongly defended Lichtenstein's work on the basis of the conceptual modifications that he made when recreating the comic panels. However, I also see no harm in crediting the original artists, as Neil Gaiman has recommended, now that norms on comic attributions have changed...Abruzzo might not be a co-creator of Drowning Girl, but a reasonable description of the work should indicate that he drew the original panel that inspired it.

Sources:

Sooke, Roy Lichtenstein: How Modern Art was Saved by Donald Duck

Foster, The First Pop Age: Painting and Subjectivity in the Art of Hamilton, Lichtenstein, Warhol, Richter, and Ruscha