Why aren't left turn signals "yields" rather than "stops"?

Jerks who partially turn into oncoming traffic in order to slowly force their way through, thus bringing the flow to a grind, probably have something to do with it. Beyond that, the same question can be asked of red lights in general - forget about left turn lights. Why are there red lights when stop signs are perfectly adequate? And why can't I go through a red if I see there's no traffic coming on the cross street? The answer is of course that stop signs aren't perfectly adequate; at some point they'd be dangerous, so red lights are put in place. And I can't go through a red light that looks safe to me because it's more important for everyone to be consistent and safe than to allow potential idiots to take it upon themselves to decide that they want to save thirty seconds.

I don't know. If I had to guess it's because highway designers want to add in extra safety measures to keep idiots from causing wrecks. But here in Springfield MO we've gotten these new fangled things called http://en.wikipedia.org/wiki/Diverging_diamond_interchange They have sprung up all over town, and my understanding is that it's a trial basis....guinea pig thing to see how they work before they go national. In these interchanges you drive on the left, making all left turns open and right turns stops. It also allows for a driver to exit a North or East bound freeway and cross the overpass to get back on the South or West bound freeway without stopping (or in some cases slowing down!). I really like them cause they speed up heavy traffic flow areas, and it's always funny cause you can always tell who is the out of towners...they get to the interchange and stop-confused-not knowing what the hell to do.

A left-turning driver may be watching for a gap in oncoming traffic and http://i.imgur.com/3zIRxXc.png.

As magnetsphere pointed out, sometimes the red is there to protect heavy pedestrian and/or bike traffic that might otherwise be missed.

I've noticed that the left turn is frequently red when the crosswalk they would be crossing through is green. At least at smaller intersections. I imagine that is because while people may look for cars they rarely look for pedestrians.

What you talking about it now being implemented in many places with the http://en.wikipedia.org/wiki/Traffic-light_signalling_and_operation#Flashing_yellow_arrow This is called a permissive left turn meaning you must yield to oncoming traffic. This is better than just having a circular green light with permissive left turn because it prevents what is called the http://en.wikipedia.org/wiki/Yellow_trap The yellow trap is when the circular green goes to yellow, people automatically assume the other direction sees the same thing, thinking you are free to turn left as the circular green goes to yellow. However this can be a fatal trap if the lights are sequenced so that the oncoming traffic is still green while you attempt a left turn on yellow. The yellow arrow prevents this by explicitly indicating that you still must yield to oncoming traffic.

There are lots of intersections that lack a distinct left-turn signal, and instead just have a sign that says "Left Turn Yield On Green" or something similar. Generally they are lower-volume interchanges. They are kind of a mess, and I think they are rightly abhorred by modern civil engineers. One of the reasons is probably that high-speed side impacts — the so-called "T-bone" collision — are really dangerous and tend to cause a high degree of injuries / fatalities. Compared to a head-on or rear-end collision, the car can't protect you nearly as well against another vehicle striking it from the side. So it follows that turning, or really any maneuver when you expose the side of your vehicle to oncoming traffic, should be done with extreme care.

We have a flashing yellow arrow on protected left turns here (after the solid green arrow protected phase of the light, it stays flashing yellow until the left turn and straight lights all go red), but not on the busier intersections. There's a particularly annoying one that gives you about 2 seconds of protected green left, so that only one car can get through protected before everyone else waits for an opening during the flashing yellow for the duration of the cycle.

When the straight-through traffic is passing, why do the left-turning lanes get a red circular light rather than a green circular light or a yellow circular light? Depends on the intersection. Some are like you describe. Others have a left turn signal indicating that the cars turning left have the right of way. But plenty of intersections have a "Left Turn Yield on Green" sign, indicating that if the left turn signal is not illuminated, cars may still turn left but must yield to oncoming traffic. Which one you get depends on the characteristics--both physical and traffic--of the particular intersection. Sometimes it makes sense to let cars turn left at will. Other times it makes sense to force them to wait for a protected turn.