Not always true. You can google about Paypal horror stories - yes, it does protect customers, but it often goes overboard.
Somewhat related: these days I'm dealing with a seller who used ebay's Global Shipping Program. He's got proof he did send the item (and I believe him), however after it was delivered to eBay's depot it... disappeared. For weeks now eBay is trying to locate who the employee who signed for it (we've got a name) is! Of course I can ask and be rightly refunded for it, but the seller seems like he's left out to dry by eBay's partner, while its other partner takes his money and gives it back to me.