@Pikka:
Infrastructure sharing does refer to - as the name implies - all kind of transports, not just rail. So it has to do everything with airports, too. Implementing that via the source code of the game offers the better flexibility in a sense to be more future-extensible (and the option to turn it on/off on a per-player and per game basis, charge fees for usage, etc). It also allows to use that for existing airport newgrfs without any modification to existing newgrfs - which is IMO an important argument as sharing is a general thing, not something limited to a certain style (newgrf) of airports. Just like stations are. But unlike abilities of industries or vehicles.
IMO implementing how things, vehicles, stations look like and how they work: a clear newgrf thing
implementing how players interact with eachother: a clear programme thing (and sharing is exactly that).
The argument to have public airports rather as an industry (part) makes to me more sense, too:
An industry is a thing open to all companies since TTO. The only thing you, as company, may do is fund an industry and doing so decide upon a place, but then your control is gone. And we have those public heliports, they're just called oil rigs.
A station (and an airport is one) is a company-owned and controlled thing since TTO.
