Right, because there is always a good line of sight to where an emergency vehicle is. Here in the real world there are large vehicles, buildings, etc. Similarly, it can "see" a car (at least, as long as it is moving), but it can't see that it is honking its horn.
edited to add:
More particularly, in reality, hearing a siren often doesn't tell you where it is. But it is a caution to slow down and be observant. Sure, emergency vehicles will virtually always cautiously negotiate intersections and so an FSD car may get by with only visual perception. And it would likely slow down if other traffic did, but what about if there isn't traffic in front of it to slow it down as it hits the intersection and green light just as the emergency vehicle enters from the side?