Police spotting (junction watch)

I would imagine the thinking is that a think red line through the symbol might make the symbol itself harder to read (especially when moving - when they were developing the road signage systems they tested the readability of them by attaching them to the top of vehicles and driving them past a group of stationary "drivers" at various speeds).

we're on a bicycle, we're not going at a breaknecking speed to not tell what's the symbol behind the red line.