At the risk of not piling on to the Apple dislike here, if you use a non-Apple payment system, and you have an issue, you now need to go to the specific company and payment system, you can no longer demand your money back by dealing with a single company. I presume even more scam apps will take your money and ignore your complaints; imagine trying to sue a company in some foreign country to try to get your money back…
Is this not what the credit card system (with chargebacks and liability shift) is supposed to counter more generally? Lots of these arguments could equally be applied to a shopping mall but we don't require all stores within a mall to use the same merchant services provider.
Edit: Maybe not globalizing App Store apps would resolve this? Or at least if you want to operate an app in a country, you need to incorporate in that country too? I think that might make it harder for overseas companies to get away with fraud.