I find it interesting that they didn't attach the buttons but the sleeve cuff itself is still essentially finished. The sewing is still there. In order for the sleeve to be shortened, there is still the labor of ripping the old stitching, pressing the sleeve creases, marking new lines and sewing them, repressing finished cuff, then attaching the buttons. This trick of theirs isn't really saving a tailor any time no really saving you any money!
It might be because it's cotton. If buttons and fake buttonholes were to be removed to shorten the sleeve, they would leave marks.