Its not a FAR as not all aircraft have radios. It is however covered in a FAA advisory circular. In addition I suspect it is a requirement in the SW ops specs making the calls mandatory.

10.1 Recommended Traffic Advisory Practices. All traffic within a 10-mile radius of a non-towered airport or a part-time-towered airport when the control tower is not operating should continuously monitor and communicate, as appropriate, on the designated CTAF until leaving the area or until clear of the movement area. After first monitoring the frequency for other traffic present passing within 10 miles from the airport, self-announcing of your position and intentions should occur between 8 and 10 miles from the airport upon arrival. Departing aircraft should continuously monitor/communicate on the appropriate frequency from startup, during taxi, and until 10 miles from the airport, unless 14 CFR or local procedures require otherwise.

10.1.1 To achieve the greatest degree of safety, it is essential that:

1. All radio-equipped aircraft transmit/receive on a common frequency identified for the purpose of airport advisories, as identified in appropriate aeronautical publications. 2. Pilots use the correct airport name, as identified in appropriate aeronautical publications, when exchanging traffic information to reduce the risk of confusion. For example, using “Midwest National Traffic” instead of the town name “Mosby Traffic” or “Clay County Traffic” at KGPH when the airport name is printed “Midwest National” on aeronautical charts. 3. To help identify one airport from another, the correct airport name should be spoken at the beginning and end of each self-announce transmission.