further to that, consider that most starship design basics were created by guys like Matt Jefferies and Ralph McQuarry-both mechanical/industrial designers-and further refined under a principle of form following function. Novel covers are created by illustrators who aren't as certain to have that sort of technical consideration in their background.
I mean when you drive a bus full of kids to school in the morning and another bus full home in the afternoon. That leaves 23 hours a day to ponder life’s little mysteries.
The original image is no doubt trademarked and copyrighted, however, those versions clearly fall under "fair use" provisions and would be seen as an homage to the original, and thus their creators couldn't be sued. As for why the artists decided to make that particular homage, who knows? It might have something to do with the stories, they might just be fans of the designs, or it might be that the artists are paid so little for their work that the only way they can make a living is to crank those puppies out as fast as they can. If that's the case, then they're not going to put a lot of time and effort into doing the artwork, and are just going to crib from whatever they can so that they can hurry up and finish the thing so they can get a paycheck.
One of them is a completely unchanged picture of the Battlestar Galactica, so that argument doesn't work.
The cover of the first TNG novel "Ghost Ship" has the Galactica upside down but otherwise completely accurate. "An Abyss of Light" has what is clearly the Galactica view head on but with what looks like some kinds of sails attached. "Death Day" has what is clearly the Galactica when viewed from the rear with a large "Independence Day" style saucer attached to the rearmost portion of the ship. Note, there are other versions of the "Death Day" cover that feature a relatively generic starship.
I would say that the cover of "The Green Hills of Earth" has a ship at least similar to the forward section of the Galactica.
Nope. Not completely accurate. There's a central spar running across the top of the ship which isn't found on the Galactica. Additionally, there are no launch bay openings on the nacelles. Finally, the craft appears to be shorter than the Galactica. Or a variation on the B-2 bomber. Looks more like a u-shaped objected designed by someone who was a fan of Brutalist architecture to me. If you're unfamiliar with that school, I've got the perfect picture for you to show you what it's like. That's a hotel in Tunisia. Many people think that it was the inspiration for the Sandcrawler in the original Star Wars.
It also reminds me of a Brutalist take on this Japanese water bus. The legal departments of the publishers would have had to clear the images before they were used, so if they thought that the covers were infringing upon copyrighted material they wouldn't have let them see the light of day.