I'm going to make a guess that it would be an electrical concern.
But, I would also assume that a pre-out would be going to an input with a "very high input impedance", so I really don't see the concern there either.
For a while, I had my main L/R driven off the normal speaker level binding post outputs, while my sub was connected to the main L/R pre-outs. It is quite a pain to make cable swaps on my system, so I used the existing L/R pre-out cables to hook up my sub when I first bought it, just as a stop-gap until I felt like pulling it all out to connect to the LFE.
I had absolutely no problems with that. Honestly, I don't think I even read the manual, I just did it and it worked fine for me on my Pio Elite.