In a similar way I can not possibly see how calling (not copying) functions from Nasal scripts would force one to to adopt the license of the source from those.
GPL says you can't link to a GPL library and license the resulting product not GPL. That's not copying, that's just function-calling. You can only bundle GPL and non-GPL if they talk via a generic interface (html, ...)
The
only difference between Nasal in an aircraft-side folder and Nasal in FGData is the directory it is in. There's no API defined to keep them apart. A different directory isn't an interface
However Python programs do not need to be licensed under the Python licence, so why would Nasal be in any way special?
The page linked by you explains that Python is special to specifically avoid that issue:
(1) GPL-compatible doesn't mean that we're distributing Python under
the GPL. All Python licenses, unlike the GPL, let you distribute
a modified version without making your changes open source. The
GPL-compatible licenses make it possible to combine Python with
other software that is released under the GPL; the others don't.
GPL-compatible is
not the same as GPL - it just means 'can be used with GPL', not 'requires you to do the same things as GPL'.
Given that the Python license makes it very clear that it is different to GPL, I wonder why the analogy was even brought up. Different licenses evidently have different restrictions.
Running (or developing) a model under FlightGear does not make that model licenced under the GPL[2]. A model does not become GPL because it has a call that could be executed by FlightGear unless that model ships a copy of FlightGear or a copy of the dependency
That's not the issue here. You can run a non-GPL model under FG as long as it talks to FG via generic interfaces. However, specifically with Nasal, no such argument can be made (unless a directory suddenly becomes an interface).
While the origin of the GPL is rooted in copyright and FG can be licensed GPL because we're collectively copyright holder, once the GPL license is valid, it can restrict not only what you can copy of the software but also what you can do with the software (case in point - google Earth is something google has copyright for, so they can license it how they want, so they can allow only certain use cases and in particular prevent us from using it to determine the location of buildings automatically for our own purposes - we're not trying to copy their code, the information itself (location) is public and can't be copyrighted - we're just not allowd to use the product to obtain it).
Similarly the GPL license can prevent you not only from copying but also from using software in a certain way.
It's just wishful thinking that licenses only apply if you want to copy something.
Our community respects author's wishes and maintaining a positive culture under which creative works can be made is much more important than wasting precious development time[3] discussing licences.
What we do as a community is not relevant for how the license works - how the license works however needs to be relevant for what we do as a community. So please stop mixing a license discussion with positive culture in our community - if X is a copyright violation, there's no point in us being positive about it, we have to deal with it.