I agree with you though that the issue is probably benign. My guess is that with large families, they were sending out 8 items and returning 6, which creates an inventory problem for a small business.
That is quite reasonable, and I think there is a chiyuv on all of us to judge our fellow Jews l’kaf z’chus instead of the other way.
Also, I think, factor in that the Torah values being careful with money (think pachim k’tanim), so when someone makes a legitimate purchase but it turns out to not be needed, it is proper to return it.
However, what may be proper for the individual turns into a headache for the seller.