Apple prices have converged quite a lot - but note that the labor content of their products prices changes very little as everything is made in same factories in China.
McDonalds (another multinational with a quite standardized product offering across markets) charges very different prices for the same product depending on local markets.
If the problem is that we want to ensure some kind of equality of outcome that is ill posed
1. Companies are not arbiters to define what that outcome should be. In fact, any such attempt boils down to the argument that the current status quo is the natural state of things and everything should stay that way. Trying to come up with a definition for local economy or local purchasing power is simply an exercise in encoding bias.
2. Plenty of products and services are priced without differentiation. For instance in the case of internet services this is probably close to 100%.
Again, all these has nothing to do with selling X hours of labour for Y hourly rate.
That's nonsensical. You can trivially find examples of where the same goods cost different prices in different locations.
Here's a really simple example. $5.79 will buy you a McDonald's Sausage Egg McMuffin meal in Montgomery AL. The same menu item is $6.71 in Los Angeles CA.
The same money has less power to buy goods and services in Los Angeles than it does in Montgomery. You know this intuitively; the same way you know that a gallon of milk is more expensive in New York City than it is in Madison.
I don't know why you're tying yourself up in knots to come to the opposite conclusion.
You can come up with your own definition of what you consider local (neighbourhood, city, county, state, country?) or cherrypick a basket you want to use as a base (McD), but that simply reflects your biases.
I don't the think premise is at all clear, never mind 'unchallengeable', especially if you take 'be equally compensated' to mean 'have the same nominal cash pay'.
Why is 'nominal cash' equivalence superior to 'purchasing power parity' from a moral perspective, for example? Is it moral for some workers to receive rewards that are richer than others (in terms of what they will buy) purely on the basis of local economics?
Why is it moral for workers in locations with generous government programs to receive the same nominal cash pay as those who might have to pay out of pocket for some of those same services in different locations?
"All other things being equal, equal pay for equal work" sounds reasonable, but when you're not holding all other things equal, the moral position is more tenuous.