It's not so much being unplugged as it is sensing whether the mains power is down. If it is, they stop backfeeding immediately, just like hardwired solar inverters do.
If this weren't done, there would be risk of electricution to anyone working on wiring in the house or on the utility distribution network.
If this weren't done, there would be risk of electricution to anyone working on wiring in the house or on the utility distribution network.