Presumably, a commercial bitcoin payment processor would spend the bucks to enter the network at a wide array of points, implementing their own double-spending detection and prevention.
This is my own presumption. One may, in a sense, view modern credit cards as a secondary currency backed by and pegged to e.g. USD. "Bitcoin credit cards" would, I expect, be the same.
The current situation blinds people to the fact that eventually, most BTC users would likely not be concerned with things like "addresses" and "blocks", they would use credit cards, bank accounts, etc. not terribly unlike they do now, trusting (hah) the finanical institutions to get it right in the background, as they have historically.