That would be illegal too, since you are not allowed to disassemble (in any way) the product (the game).
And the entire game is probably regarded, legally, as one entity, if you extract anything from this entity, such as a file, you are smoked. But of course they wouldnt go after you, mostly because they would never find out, and if they did, you are no threat to them, their product, marketing, bla bla, in any way.
OFPEC on the other hand is one of the most popular OFP communities (if you may), which makes it high profile, and hence an easy target.
Im not telling you not to use resources from other games to enhance your mission/addon/whatever.
Everyone does.
But it would be foolish by an organisation such as OFPEC to openly distribute such resources.
But enough with this legal crap already