You're talking about retailers.
I'm talking about manufacturers.
Retailers have a substantial initial markup (e.g. 50% on clothing), and a business model that allows for large markdowns (hence the 20%, 30%, 40% off sales). And after a jacket is returned, they can put it back on the rack and sell it as new.
Apple has a smaller gross profit margin (around 25% for MacBooks) and cannot resell the returned machine as new. With the discount that it has to offer on refurbished machines, plus the labor to refurbish/repackage/restock, plus the credit card processing fee, its profit on reselling the returned machine would be pretty low (maybe 5-10%). Not such a sales success after all.
Would allowing consumers to try out a MacBook, change their mind, and freely return the machine really help Apple's business? It has probably studied this and concluded "no". It probably decided to allow some returns and draw the line at "custom" configurations because, well, you have to draw the line somewhere.
If Dell, H-P, Lenovo, etc all permitted consumers to freely return ill-chosen PCs, then you might conclude the economics favor such liberal return policies.
AFAIK they don't.
If you want to buy a PC with no-question-asked return privileges, look into buying from a Best Buy or similar retailer. They usually follow the more liberal "retailer" return policy.
P.S. Would it really make sense to return even a non-custom MacBook? If I read the Apple policy right, there is a 10% open box fee, say $300 on a $3,000 computer. She might be able to simply resell it for a better price.
P.P.S. I checked Dell - they let you return
unopened hardware, no mention of returning opened/used hardware. Just a comparison point.
http://www.dell.com/content/topics/global.aspx/policy/en/policy?c=us&l=en&s=gen&~section=010