I wouldn't know the logic. But you can suggest EA to change it:
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Actually, a really good place to post requests like this is to our Build Mode Feedback thread. We have an index of requests there, so that things don't fall through the cracks. As for half walls, they have some characteristics of walls but also some of fences...it was as though their implementation was somewhat confused. Even though the half walls themselves have been designed at separate heights, the available wall height levels in the game haven't changed, and it is these levels to which the roofing is tied.
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