The premise is false.
Warning a citizen that there is a speed trap is not warning a speeder, but a citizen, that a speed trap is ahead.
Of course, cops think the other way around, the citizen flashing his lights is "obstructing justice" because all citizens speed and the warning prevents justice from being served, revenue being collected, quotas being filled. The federal court decision smacks LE on the back of the head on that score.
This is a small ruling, yet very significant, fortunately LE will not "get it."