Minimum distances for coach travel: why?

I see that some pre-Amtrak railroads required minimum distances for coach passengers, at least on certain trains:

1. Illinois Central included the following in its Magnolia Star timetable:

"Passengers entraining and detraining at points north of Memphis must travel to or from Memphis or beyond."

2. In the 1970s, the Southern Railway required coach passengers on the Piedmont (New York-Atlanta) to travel over a minimum distance- maybe 100 miles?

Why would railroads do this: solely to reduce ridership?

No. It was a way of keeping coach seats available for long distance riders who were paying higher fares. Those were the days before computerized yield management.

And on those routes there was an assortment of other trains the "shorts" could take. With Amtrak, one train has to do it all--and lucky to have that.