I think it is possible to oversimplify many conflicting strands.
Right now it is the Christmas Season in the US (a selling season with capital "S" the likes of which may be unfamiliar to some people elsewhere. Dunno) But just at the moment stores get stuffed full with almost anything the proprietor thinks will sell. Calendars, planners all sorts, are a must at the turn of the year, as are boxed Christmas (or seasonal?) cards for obvioous reasons.
As for puzzles, I seem to remember that it was thought they would appeal to the intellectual interests of people who were (probably) already intellectual, namely bookreaders, so the tie-in is not as vague as it might seem. Whether the merchandising gamble succeeds or not is another matter.
I always apprecciated the selection of pens and pencils at Borders, and now B&N, because I always arrived with a book to read but having forgotten to bring along a pen or pencil for marking. So I accumulated quite a selection, one by one.
A week ago we found ourselves in Bay Saint Louis on the Gulf coast, accidentally more or less, and on the way to going elsewhere, when we found a wonderful small indie bookstore. It was run by a nice young gentleman and it had a fabulous selection -- onesies and twosies of almost all the authors we usually think of buying. That fellow had a keen eye for picking books and authors! So we walked out with a big shopping bag for (only) $165. But the relevance for this thread is that even he also had "extraneous" stuff in addition to books.
Even amazon. the original citadel of books, sells all sorts of other merchandise -- paper towels to cameras, oriental noodles to computers.
I expect that what sells in a bookstore will stay, what doesn't won't. Even the bookstore, if it doesn't manage.
I think the extraneous stuff is a symptom, not a cause. And, among causes, I wouldn't ignore the fact that the US has just come through a withering recession (if it has) and everyone is scrambling to make ends meet -- bookstores, house sellers and realtors being just some examples that come readily to mind. At least from where I see it.
How many here would open a "pure" bookstore right now?