Cliff hangers are named thus because they evoke a sense of danger, as if hanging over the edge of a cliff. When used between chapters they create the "page turning effect" sought after by thrillers and other high octane genres. When used between books they can become distasteful if used improperly.
At that point, it's no longer a question of creative intent but business intent. To end a book on a cliffhanger is to presume that the reader would buy the next book simply to find out what happens next, instead of the book's inherent quality. This is especially the case if it is tacked on at the end for the sole purpose of a cliffhanger instead of events leading up to something meaningful. It's like pushing a reader off a cliff and refusing to hand them a rope unless they pay for it.
I also call this "The Missing Epilogue" because the problem could be corrected with a short epilogue. Something that shows the aftermath of the climax and closes up plot threads. It could even be a preview of the next book to increase anticipated sales, as long it closes the first book at the same time as opening the second.
It's about that sense of closure. For however many hundreds of pages, the plot is developed and brought to its climax, the highest point. After that, a reader expects some falling action and resolution. It doesn't have to resolve everything neatly (I've heard some authors don't like that for some reason, either creatively or out of a desire for realism) but I want to see the fallout of the climax. Don't stop at the tail end of the climax because the reader wants to see more. To exploit that desire with a "buy the next book" line is a business decision and likely to antagonize the reader.
Indeed, as a volunteer book reviewer I've read several books like that. They cut off at the tail end of the climax or introduce something new on the last page; some last second twist to stir the reader into a frenzy and buy the next book. Instead, I've thought "Ick!" and downgraded their final rank in my review. I don't read sequels to books like this because they are likely to have similar endings, final book included.
Driving sales can be the only deliberate intention of a goading cliffhanger. If someone denies this then it means that they are sloppy, lazy, and/or lack the skill to resolve the conflict they created. Conflict is created, developed and then resolved; that's the basic structure of a story. I don't want to hear any "I have to leave it unresolved for the next book!" whining. A sufficiently skilled author can close a book's conflict while leaving the series' conflict open.
Here are four examples of such writers:
1. Isaac Hooke did this for his "Forever Gate" series. The first one, for instance, ends the main narrative with a shocking twist and thus a cliffhanger because the reader does not know what's going on. However, the preview of the next book showed the aftermath of that twist and at the same time queues up the second one. This way the reader is not left hanging about the conclusion for the first and is instead excited for the second.
2. J.K. Rowling did a fantastic job of this in "Harry Potter". The conflict of the series was always about Harry clashing with Voldemort but the individual seven books each had their own conflict that was set up and resolved between the covers. The first book, for instance, has Voldemort attempting to restore himself with the Philosopher's Stone. While Harry prevents him from getting it, Evil Plan foiled, the evil wizard is still out there and he can try again.
3. Ranjit More does this in "The Underworld King" a little differently. There are two main conflicts; demons vs snake monsters and demons vs gods. It starts with the first and is primarily concerned with the first but the second is a significant plot thread. By the end of the book, the first conflict is resolved completely and the final lines signify that the second will be prominent in the next book. It's like shifting weight from one foot to the other.
4. The "Inspector Gadget" cartoon series. Every episode began with some Evil Plan by Dr. Claw and every episode ended with him shouting "I'll get you next time, Gadget! NEXT TIME!"
By using similar techniques as these four examples, a writer can engage their audience to a deeper degree and get them excited for the next adventure, without angering them with goading cliffhangers.