American specifically prohibits the practices commonly known as:

Back to Back Ticketing: The combination of two or more roundtrip excursion fares end to end for the purpose of circumventing minimum stay requirements.

Throwaway Ticketing: The usage of roundtrip excursion fare for one-way travel, and

Hidden City/Point Beyond Ticketing: Purchase of a fare from a point before the passenger's actual origin or to a point beyond the passenger's actual destination.

Duplicate and Impossible/Illogical Bookings: Duplicate or impossible/illogical American Airlines bookings are prohibited without prior authorization from American Airlines. A duplicate or impossible/illogical booking includes, but is not limited to, bookings for the same passenger on flights traveling on or about the same date between one or more of the same or nearby origin and/or destination (such as JFKDFW and LGADFW or DFWLAX and DFWONT), or bookings with connections that depart before the arrival of the inbound flight.

Fraudulent, Fictitious and Abusive Bookings: Fraudulent, fictitious and/or abusive bookings are prohibited. These types of bookings are defined as any bookings made without having been requested by or on behalf of the named passenger. Additionally, creating bookings to hold or block seats for the purpose of obtaining lower fares, AAdvantage award inventory, or upgrades that may not otherwise be available, or to circumvent any of American Airlines' fare rules or policies, is prohibited without prior authorization from American Airlines.

Where a ticket is invalidated as the result of the passenger's non-compliance with any term or condition of sale, American has the right in its sole discretion to:

Cancel any remaining portion of the passenger's itinerary,
Confiscate unused flight coupons,
Refuse to board the passenger or check the passenger's luggage, or
Assess the passenger for the reasonable remaining value of the ticket, which shall be no less than the difference between the fare actually paid and the lowest fare applicable to the passenger's actual itinerary

3.3.3.1. In the event that you do not use the return trip shown on the Ticket, and Art. 3.2.3. not withstanding, we shall recalculate and demand the price in accordance with the one way fare for the leg actually flown valid at the time of first issuance of the Ticket.

I live in city A and work in city B. A and B are about 230 miles apart.

The lowest one-way fare from A to B is usually around $497. (FOUR-HUNDRED AND NINETY-SEVEN)
The lowest one-way fare from A to B to Newark is $95.

There is only one airline serving A to B.

So, for that last several month(since I started working in city B), I've been
buying tickets for flights from A to Newark. I always get off in city B and
drive home. On the return leg, I usually take Amtrak, which only charges
$60, but takes several hours for the same route.

I do this every week, so the savings are very BIG.

My mistake is that I did enter my frequent flyer airline for that airline and
they caught my little trick. They sent me a letter warning me that it's
against airline rules.