Just wondering (and based on guessing about the physics) - this might be to do with the way the frets are attached. If they were glued in at each end of the fret but not in the middle, you might get a "ringing" of the fret itself - which the strings wouldn't affect, so you could change the strings without affecting the sound. And since this might allow the manufacturer to halve their fret glue budget, it's the sort of thing they might try.
As I say, that's a guess.