Even the VM2 has this, and it uses the same body style, same bridge pickup, same everything. I don't know why they don't do it for the standard VM either.
One possible reason that occurs to me now that I've done it: The P pickup in the VM5 is extremely powerful and cuts through just about anything, so Sandberg may be worried that users would complain about a perceived volume drop when using both pickups and splitting the humbucker. On the TM basses, I assume the neck pickup might be a little weaker, and therefor balances better with both the full humbucker and the split one.
I haven't done any proper analysis yet though - I'm not sure if there is an actual volume drop between the P/humbucker combo and the P/split combo. It might also be that it's just a perceived volume drop because a lot of the midrange gets scooped out in that center position. Most likely it's a combination of both.