Thrilled you asked! So yes: Treatment is always required, but the final destination of the treated water can vary. For instance, in a lot of places they may have municipal water TO a home or business, but that may be discharged to septic, as opposed to the river. Also in a lot of areas, water may be taken out of an underground aquifer (either by private well or a municipality) but when treated it may be discharged into a river or ocean. That can create problems because if you're near the coast, the empty space in the aquifer may be filled by salt/brackish water that can lead to salinity rises in the aquifer. To solve that some places turn to "ground water recharge", which is just a fancy way of saying "we built a big well to put it back in the aquifer".
Increasingly, you're seeing some places essentially sell their treated water. Santa Rosa CA, for instance, built an entire pipeline that goes from their treatment facility to another municipality to be injected into their groundwater.
So yes, everywhere treats it, but the final destination makes a difference. Las Vegas (or anyone else on the river) only gets credit for what goes back into the river, so any evaporation etc is a problem. It sounds trivial, but there is a reason those other strategies exist. It essentially doubles every pipe, limits where you can park a treatment plant etc. Vegas also does some great grey water re-use. That essentially means it doesn't go "back" but can get used many many times, limiting the initial draw.
Wastewater is funny because it's far from rocket science, but the numbers to implement any of it get staggering very quickly.
