People want them to conveniently power their house during an outage. Plug one end into a generator, the other into a random socket, and poof! You have power (so long as your house isn’t drawing more than whatever breaker you’re plugged into)

Problem is unless you turned off the whole-house-breaker, you are now feeding electricity back upstream into the grid. This is very bad. The friendly linemen who are working to get your power back on can’t de-energize the lines they’re trying to fix and will have a hell of a time working out which house is causing the problem.