The battery bit is probably AC not wanting to guarantee that you'll have access to plug-in power. And AC doesn't want something bad to happen, and get blamed if you couldn't plug-in because your socket wasn't working. Quite possibly their purchase agreement for the power system specifically prohibits medical devices, so they'd stand no chance in court.
Possibly AC had an (unpublished) incident in the past. Or their lawyers are dreaming up edge cases to rack up hours.