Agreed. However as you rightly point out, this is data. You're not statically linking it as you would for example a library. In this particular instance I don't think the X-Plane people have given a great deal of thought to the suitability of the license to the type of material they wish to protect.
Combining propietary and GPL software has been done for many year by mobile phone and set top box manufacturers, it just requires a little thought during the implementation stage in order to ensure that whilst the 2 interact, they remain very much separate entities. A proprietary application and a GPL database would be one such example. Running CDDL licensed Solaris filesystem drivers in user space under a GPL Linux kernel would be another.
Anyway, I'm sure you've got other plans so it's most likely a moot point.