I am a surgery resident. This "rule" only applied to first year residents, and was started in 2011. There in fact is a study being conducted where one arm of first year residents follows the 16 hour max and another follows the 28 hour max. It seems the results from this trial suggest the 16 hour max is detrimental to learning, hand-off logistics and scheduling.
Regardless, after your first year, no matter what program or specialty, 28 hour calls become the norm. As a second year surgery resident for example, you can plan on taking a 28 hour call two or three times a week. This is just how it is. For two main reasons:
1) There are not enough of us for everyone to get 8 hours of sleep a night. People can debate why or ways we should fix this, whether it's widening scope of practice for nurse practitioners or decreasing our pay so there is more of us, but logistically there are not enough residents to keep everyone well rested.
2) There is physiological relevance to a 24 hour cycle in many diseases. For example, if a patient comes in with a bowel obstruction, the process of observation, lab evaluation, imaging, +\- surgery takes sometimes 24 hours. 12 is sometimes not enough to appreciate the full evolution of acute disease.
Big topic here. My experience is anecdotal and limited. Why they did a big study to see what was best. Still, it only applies to first years.