This applies to the United States: It makes sense that a TV show that
is scheduled at, say, 8 p.m. in the Eastern time zone and Pacific time
zone shows at 7 p.m. in the Central zone. After all, it's easier (or
so I would think) to present a show to everyone in the Eastern and
Central time zones simultaneously. So I understand why shows in the
Central zone are shown "early." But why is that also true for the
Mountain time zone?

In other words, why is a show that is advertised as "9 p.m., 8 p.m.
Central" also shown at 8 p.m. in the Mountain zone?

Thanks.

As I understand it, the TV networks have only two broadcast schedules,
an Eastern one and a Western one. Obviously, the Central and Mountain
zones must get one or the other. Central gets the Eastern feed.
Mountain gets the Western feed.

This may be helpful:

"Dual Feed The use of two transponders to broadcast programs.
Typically one feed for the Eastern and Central time zones (called the
eastern feed) and the second feed for the Pacific time zone (called
western feed). The Mountain Time Zone is divided in two with those
areas east of the Continental Divide receiving the eastern feed and
those west of the Continental Divide receiving the western feed. One
of advantages of a dual feed means that a network's prime time
programming will air 8-11PM on both the east and west coast. All pay
cable networks (measured by Nielsen), use a dual feed. A growing
number of ad supported cable networks also use a dual feed."

http://www.horizonmedia.com/glossary/d.htm

You have asked a very good question, and the reason you haven't gotten
a definitive answer is that there isn't one.

Pinkfreud-ga has given you some good information about satellite
feeds, but the distribution arrangements of broadcast networks and
cable networks vary in the number of feeds and in the contractual
flexibility of affiliated broadcast stations and cable systems to
choose the time slot for a particular broadcast.

From the regulatory standpoint, a television station in the Mountain
Time Zone can choose whether it wants to use 8-11pm or 7-10pm weekdays
as "prime time" for regulatory purposes. And, the feed that a
particular station gets from its network doesn't in itself determine
that choice, since the early (Mountain Time) eastern feed of a network
can be conveniently taped for broadcast later in the evening.  Also,
the contractual arrangements between a broadcast network and its
Mountain Time Zone affiliates may affect their discretion to choose a
time frame for the network schedule.

Finally, Mountain Time Zone issues have always been an afterthought
for regulatory and network distribution purposes because that time
zone includes only about 6% of the population of the "lower 48" and is
split by the Continental Divide into stations and communities more
oriented to the east or the west.

Thanks for your comments, Pinkfreud and MarkJ.

What Pinkfreud says is correct with regard to some cable TV channels.
However, the broadcast networks seem to have three feeds (or whatever
the term is), not counting Alaska/Hawaii. There's the Eastern/Central
schedule, the Mountain schedule and the Pacific schedule. So that
means that shows that are seen in Denver and Salt Lake City (and all
others in Mountain time, as far as I know) an hour after they are seen
in New York and Chicago, and two hours before they're seen in Los
Angeles. It doesn't seem like the logical way to do things, which is
why I guess I'm asking the question.

As it happens I was the consulting engineer responsible for creating
and installing some technology related to this question, at least at
NBC, CBS and several of the cable networks.  Each network decides how
many regionalized programming feeds to originate and distribute to the
various time zones, and this is mainly a financial decision, not a
technical one.

The tech I installed was designed to continuously record and later
replay (to one or several "output" time zones) the original feed,
normally eastern time of course.  But, for example, at that time CBS
(from their New York center) would originate some 20 separate weekend
feeds simultaneously, since different regions viewed various sporting
events.

Anyhow, the short answer is that it costs extra money to uplink and
distribute each feed (for a given time zone's schedule), and there are
simply too ew viewers in Mountain Time to justify a separate feed to
those affiliates--not enough extra ad revenues from shifting the
Central or Pacific timing by the hour.

Later, the time delay equipment got much cheaper to deploy (recording
and playing via hard disk vs. the original banks of expensive
videotape machines) and individual affiliate stations began to make
their own time-shifting/scheduling decisions in certain markets.