OK, one last time. Imagine there are 3 signs in a row. In the order you see them:-
"BIKE LANE CLOSED AHEAD"
"BIKES USE MAIN CARRAIGEWAY"
"WATCH FOR BIKES"
Without the context of the first two the third does look stupid.
Or, since it's an electronic sign, it may loop through these 3 messages every few seconds. Then it does make sense.
But, never let the (possible) truth stand in the way of an Internet meme.