Your presuming that there is exposed access to the cable feeding the main panel, that is often not the case. I have no problem opening a mains panel or otherwise mucking around to get an inductive clamp on such a cable, but I don't think you can make the assumption about their target market. Their solution may seem 'clunky' from a technical standpoint, but it has one very important thing going for it: anyone can do it, without risk and without a knowledge of electricity, all they need is a conventional meter.

Counterpoint: the fact that they need an outlet to power the thing is bad, now the 'common Joe/Jane' doesn't need to mess with the electrical stuff but instead they need to drill a hole through the wall, near the meter, to get a wire to the sensor. I don't like the idea that the user to has a drill a hole near the mains entrance to their home, that can result in bad things.