I think about this every morning when I get in the car and I can't see out the back windshield.
Here is the rule regarding windshields in general. Windshield wiper systems.--The windshield on every motor vehicle other than a motorcycle or special mobile equipment shall be equipped with a wiper system capable of cleaning rain, snow or other moisture from the windshield, and so constructed as to be controlled or operated by the driver of the vehicle. Now what this doesn't say is that it is not stating where the windshield wiper should be. The reason I believe that a wiper does not have to be on the back window, is because of where most drivers attention is. As a driver most of the time, your concentration is on the front view. You are only allowed to look to the rear view for no more than 2-3 seconds. Basically my point is because of the time of concentration it isn't something that is required. If it were so much of a concern it would have been made law by now.
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What type of car do you have? In Europe most hatchbacks, estates (station wagons) and SUV's will have a wiper on the rear window. It's only saloon cars (sedans) that don't. I've owned two saloons, one hatchback and am about to take delivery of my first estate. The saloons don't really need a rear wiper because of the aerodynamic effect and the boot (trunk). It doesn't matter how hard it's raining outside, driving along at anything but the lowest speeds will generate enough of a slipstream so that no rain at all will fall on the back window. In fact, even under the worst weather conditions I've found that you get a far better view out of the rear than I ever had with the hatchback. The problem that the hatchback had was the spray from the wheels would be sucked back against the rear of the car and liberally cover the rear window, which was when the wiper really came in handy. However, what you're talking about seems to be condensation on the outside of the car. Really you should deal with that before you get in and drive, together with any condensation on the side windows and mirrors. It's only takes a minute to go round with the rubber-bladed side of a handheld ice scraper and clear all your windows.
I know that vehicles that have two side mirrors don't need a rear window at all (pick ups and vans etc). If not all vehicles do not have rear windows, I don't think that they would make it standard for rear wipers. This is considered a luxury not a necessity. I think that the SUVs have them more often because of the angle of the window and no trunk. When driving forward, cars have a more angled rear window and moisture and snow doesn't stay on it as much as a SUV's rear window which is more up and down. This causes the wind to wrap around the back and blow the rain and snow onto the window.
