My rear brake was very weak when new. (I could get more braking effect by letting off the throttle than I could by hammering the rear brake.) Even to the point that I suspected the dealership hadn't cleaned off any packing grease meant to prevent corrosion during shipping. Not sure there was any grease, but the rear brake really sucked.
Eventually I pulled the pads and scuffed them with sand paper (keep it as even as possible and I could see a change in their surface as they were sanded). Also cleaned both sides of the rotor with brake cleaner. Seemed to be better right away, and kept getting better with use.
The rear brake seems pretty normal to me now and it will lock up. But in the early days I thought the same thing as you do now.