I've never bothered asking, but I assume the electric MTBs and fat bikes you see on the market are not restricted in the first place if they're for off-road use? It's not much of a suprise they're starting to appear on the road.
They are restricted, the Law doesn't differentiate between on-road and off-road use, it simply says that pedal assist cycles must be limited to 15mph. My Dad has one, strangely enough our rides average 15mph.
On this score I'd be quite happy for people with de-restricted bikes to have them confiscated, if it was regularly in the News people might think twice.