Very good question.
There are two types of emergency braking, when you pull the emergency brake lever:
1. The rear caliper activates. This is a less expensive way to provide by motor company as all it needs is a linkage.
2. Rear drum brake in which the rotor is for normal braking by caliper, but also is a drum brake for emergency and parking, the drum brake is a separate mechanism from the caliper. So this type of setup is a combination of brake caliper and brake drum as the rear brake is a much safer set up than just caliper (emergency) brake, which can be seen more on true high performance import cars like EVO or Subaru WRX and STi.
See a typical design of rear rotor without drum brake Vs. EVO and Sti rear drum brake. (see the difference of deep diameter for drum):
Our design philosophy is safety first. In other words, we will not alter the material for its intended performance nature i.e. we use only cast iron for drum brake like OE, not aluminum. Although aluminum is easier to be made available but is not the right material for braking surface. We either do it right (even costs a lot more and harder to make) or don't do it. See our answer to STi forum for the similar concern.