I suspect that the fan is wired that way so that the fan can be plugged in to the UPS during an extended outage.
Is there a transfer switch at the UPS output, or maybe built in to the UPS to allow the load to be powered while the UPS is taken offline for servicing?
Is there a transfer switch to allow the UPS to be powered by a generator, separated from the building power? That could also explain the fan wiring, since during heavy charging is when you would really want to have that fan operational.
