Block heaters are also super cheap aftermarket, usually $40 or less. Most are made by Philips and Temro, including most OEM heaters. The aftermarket ones made by Philips and Temro are sold under the brand name Zerostart. They are literally nothing more than a 600 watt water heater element, a freeze plug mount, and a standard 120V cable.
The block heaters do not interface with the PCM or computer at all, and the PCM has no idea one is or is not installed. Mine did not come with one (though I added one aftermarket), and also did not have a battery temperature sensor.
Interestingly enough, the battery temperature sensor logic in the PCM regarding charge voltage adjustment was still active, causing my charging voltage to sit at 13.4V all the time. The PCM, in the absence of a sensor, merely assumed that the battery temperature was always maxed out, regardless of how hot or cold it actually was. (Pretty easy way to kill an AGM.) I went in with a tuner and flattened the curve and now charge at 14.3V.
It might make sense as to northern market TJs being more likely to have them, as in the PCM logic, charging voltage is ramped up from 13.4V all the way to 14.7V as the temperature decreases. This normally would allow the (flooded lead-acid) battery to charge more effectively in cold weather, while protecting it in hotter weather. In warmer climates, I could see an argument that the lack of extreme temperature changes could make it not cost effective, or that the lack of a sensor probably did not kill a battery as fast as it would in northern climates.
Essentially, it probably boiled down to a cost-vs-return selection. If Chrysler hypothetically saw that the lack of a sensor would likely result in the battery failing under warranty, one might be installed. It would be cheaper to add a sensor versus warranty a battery. But if they could get away with doing neither, they probably would.