Differences in frame seat tube angle through the size range are not just for KOPS. It is also done to keep the fore-aft center of gravity relatively constant through the size range. But, there are also plenty of manufactures who use 73 degree seat tube angle though the whole size range and let people fiddle with seatposts if the want too.

I think associating crank length with leg length just makes sense, but I think it most likely benefits the short riders who have been using cranks that are to long more than tall riders who have been using cranks that are to short.