Sounds like a kludge. Many subs have a pass-through, with or without filtering the audio. This sort of feature almost makes sense in an old-school setup (before the advent of the modern digital receiver), where you'd cobble together your own satellite/sub system.

When you're moving to a modern 5.1 world, you want to run a separate line from the receiver to the sub. If for some reason that's infeasible, then pretty much the only other option is to tell the receiver that (a) you don't have a sub and (b) your front speakers are "small" while your rear speakers are "large". This will cause the receiver to direct all the low-frequency information to the only place that can handle it: the "large" speakers. (Or, it wouldn't surprise me if some subset of the receivers get deeply confused if you set them up this way.)