The fee is usually not returned to the merchant from payment provider in case of chargeback (in this case, merchant is Apple, but payment gateway may also not get the fee returned from acquiring bank also). In this case, Apple returning the fee result in them paying the refund out of their own pocket, which may be a source of abuse. This varies by payment provider, and some do charge extra in case this happens. I've posted a bit more about this in other subthread.[1]
Because Apple might not be able to "take away" an app that was installed and then charged back instead of returned?