Why is it "common business sense" to not sell black cats in October?

Your supposition is correct that in America, there is a fairly commonly held belief that people will obtain black cats around Halloween in order to do them harm. Some shelters may even restrict black cat adoptions for this reason. However, the statement that this is "common sense" advice is fallacious. I am American, and I never heard of such a thing until I saw it being discussed on the Pets Stack Exchange. Furthermore, the idea that people harm cats around Halloween is most likely a urban legend, as stated on many websites such as Snopes, though it is true that shelters very well may limit adoptions because of it.

Honestly, it's extra strange to me that such an urban legend persists when the American culture around Halloween seems to be changing pretty radically over the past several years. It used to be that kids would go out trick-or-treating to private homes, with entrepeneuring older kids going unsupervised into the late hours trick-or-treating and pulling pranks or generally getting up to mischief, but these days door-to-door trick-or-treaters are becoming more and more rare in favor of going to events deemed "safe."

People can get pretty weird ideas about things, superstitions and such. Some get ideas that black cats are bad luck, or an evil omen there is said to be an increase of black cats sustaining injuries around the Halloween holiday by individuals, and reports of missing black cats as well. People like to obtain black cats for whatever purposes around that time of year, maybe for novelty or dark reasons. It is just safer for black cats not to be put in jeopardy to potentially bad situations because there all sorts who have no scruples.