mmodri: Oh duh, stupid me. I didn't even consider checking that. I just assumed they were in their own OS category. Weird that they're extras. Disregard my previous claim :D
As Gydion said, it is because they are not officially (supported) versions. Some other games also have e.g.non-English versions of games in the extras (instead behind a separate language category) because they are fan-made translations of the game, not officially published by the original game publisher.
For instance, Planescape: Torment has these alternative language and OS versions in the extras, even if you are downloading only the English Windows version of the game:
Windows Russian localization (Fargus) 1100 MB
Mac Russian localization (Fargus) 1200 MB
Linux Russian localization (Fargus) 1300 MB
I guess the only way skipping downloading them is to use the blacklist feature, but then you have to be careful that you skip only the correct files and not something you didn't intend. And as there are quite many such games having these "alternative versions" in the extras, you may need to add quite many entries in the command line command. Also, I am unsure if e.g. clean and verify commands yet support the blacklist feature?
I recall at least some of the Broken Sword games having such alternative versions (Linux and Mac versions of the Classic version), and even The Witcher 3 (and GOTY) have quite big non-English Goodies Packs for various languages in the extras.