Looking at it from the Scamerati's viewpoint, I daresay they perceive a centrally mounted weapon as more versatile and less prone to direct attack from the captain gatso brigade.
The quid pro quo is that, once nobbled, they will need to seek a greater level of permission to fix or replace them as it will involve substantial carriageway disruption. Sooo... thinking caps on...
Paintballs would be one form of countermeasure, delivered A-team style from behind the driver

Unlike those on the roadside which can be wiped clean with a turps rag, cleaning off one of these would involve roping off two fast lanes of motorway for probably the best part of a mile each side.