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.

Thanks. I would have asked Claude or something but I know so little about this topic that I wouldn't have been skeptical enough of its answer.