Odd indeed.
I was under the impression that ZED's were implemented :
A. Uniform fare calculation
B. Eliminate the necessity for multiple back ups

In prohibiting the use of ' co terminals ', it defeats 'B' which will lead to multiple back ups again, thus increasing the work of ticket agents
and at the back end, refund agents.

At UA we have very few interline E ticket agreements & none of these contain such a restriction.