Don't quote me on this, but it's probably because of this: Let's say they advertise your system will be battery powered for 10 minutes on the UPS with a certain monitor etc. If you plug a surge in with all your computer stuff etc. and your system goes down in 1 minute, you'll call tech support and say "my battery isn't working right." It's probably just to avoid stuff like that I would think. It won't hurt anything to plug a surge into it, but don't expect the battery to last as long as advertised.