Those airlines don't provide a better product for a lower cost, they are selling you a higher-end product and billing part of it to their own citizens. That's what government subsidy means.
The US subsidises gasoline quite aggressively when compared to other countries around the world. You can see this in fuel costs; routinely US gasoline used to be cheaper by the gallon than Australian gasoline was by the litre. I've never once heard an American complain that their fuel is too cheap despite the fact that their taxes and government are directly responsible for said low cost.
For the person you’re replying to, it seems clear that the most important thing about the word “cost” is how much they personally pay — third parties are irrelevant.
“Sound”. “Cost”. The meanings of words are not as precisely identical as our subjective feelings tell us.