the only reason i can come up with is that if you do not want sub output, you can always turn off your sub. This would give you the same results as a pure pass-through. Some people do not care about double-bass and simply want their sub to be doing something regardless. So i guess having the output always summed to the sub gives you the choice.

that being said, i also wish they did not do it that way. It's especially odd for outlaw because using their ICBM along with the 950's multi-inputs is a useless configuration...there is no way to get proper bass management using these two devices (the multi-inputs only, the icbm works fine with all of the other features of the 950) together.