You need to add up ALL the costs of keeping 2 separate sizes of each color of band in the whole supply chain, from factory to retail store. Don't forget that every retailer will need twice as many pegs to hang them on (if they want to keep every band in stock). Add in the lost opportunity cost of the OTHER items the retailer would have had space to sell, if they weren't selling twice as many different sport band SKUs. Add in the cost of the uncertainty about which band size will be most popular -- if you're not sure, you have to make too many of one size if you hope to have just enough of the other size. (What will eventually happen to all the excess of the size you made too many of?) Add in the cost of the confusion and dissatisfaction of customers who aren't sure which size to buy, or who make an extra trip to the store to exchange for the other size. (Any bands returned by a customer will almost certainly be thrown away; they aren't worth the cost of cleaning and repackaging that would be required to sell them as refurbished.) Add in the cost of the extra staff time required to help those confused customers, or the cost of the unhappy customers who have to wait longer because the staff is busier than ever.
Against all of the above considerations (and probably more), compare the cost of using a little more raw material and putting both sizes in each package.
Apple only sells something if the margin is high enough to make it worth their trouble. Have you noticed that Apple retail stores sell almost nothing that costs less than $19 or $29? It's not because they couldn't sell a $5 item profitably, it's because the $5 item would replace something more profitable or more popular, and they only have a fixed amount of space. What if it turned out that a single-size sport band needed a retail price of $39 (certainly not $25!) to justify its space in the store, and to offset all the extra cost in my guess-list above? Wah! Apple is ripping us off!
Apple has done the calculation, and they think they chose the better way. Sorry to be blunt, but I trust their judgement more than yours. But you can prove Apple wrong -- all you have to do is offer bands packaged the way you say Apple should have done it. You have to meet demand at least as well as Apple does. And you have to make enough profit to remain in business.