It'd probably be easier for a lot of people if the funds were automatically refunded, but (and as much as it pains me to say it) I dare say it'd be more trouble that it's worth for eBay, mostly because the fees charged aren't always paid for when they're reversed due to NPB's, plus people use a wide variety of payment methods. With what would probably be a relatively high volume of reversals on a daily basis, they'd have to work out what fees have been paid for, and how, then issue individual refunds.