To be fair, most advanced nations air travel is heavily subsidised by 3rd would countries. If people had to pay the real cost of their ticket, very few people would be able to afford it.
In what way? It's definitely subsidized by the local and national government (e.g. air traffic control, construction of airports), but how's it subsidized by foreign countries?
If anything, it seems like third world countries would be subsidized by first world countries (not that I think that's the case; just logically).
I'm assuming the poster thinks that because airfares are very cheap in a massive competitive market like the USA and 10-100x as much/mile in a small highly regulated 3rd world market then somehow it is being subsidized.
There is effective subsidy the other way around. Whatever the lawyers say, both Airbus and Boeing are subsidized by their governments in their own ways. All the R&D of aircraft, engines, radar, navigation was paid for by the defence budgets of superpowers.
And flights to most 3rd world countries are directly subsidized by governments - at least for national airlines flying to former colonies.