It’s covering their legal asses. Kinda like those stupid “speed checked by electrical and other devices” types of signs on the highways - well, duh! There are a gazillion of these stupid signs (and more every year) popping up everywhere because somewhere, in some single court case, some jury said “fuck the power” and gave the defendant a pass over some incredibly flimsy excuse like “well, there wasn’t any notification that they were using a radar, or I wouldn’t have been speeding.” Bullshit! Who doesn’t know cops use radar?