activating a dongle is the same as twisting the spoke magnet until it starts reading wrongly, except that the dongle has a set error percentage of 50%.If I was stopped for doing 85mph on a 70mph motorway but my speedo was reading 69mph I don't think I'd have much of a case.
Or another example, if somebody changes diff ratios or tyre sizes on their car their speedo will read incorrectly but it's still their responsibility to make sure that they don't exceed speed limits.
If you have a wrong speedo, it's like after you activated the dongle. If you don't activate your dongle, you are not speeding.
The situation is similar to someone going to a demonstration with a flag pole, it's legal until you poke the pole into somebody's face.