Xeus, Here on the east coast seismic usually doesnt control the design, wind does. Perhaps it is just a conservative approach by assuming pinned behavior, and adding in shear walls to handle lateral forces with the moment resistive nature of the frame as sort of suspenders along with a belt.
If there are no cold joints in the concrete system, it is inherently fixed at each end. Think of a simple foundation in plan, really the whole rectangle in plan is a rigid box
