I looked into this a bit more just now and it seems like maybe the mains power synchronization was more about ease on the camera side than for the receivers:
"The NTSC field refresh frequency in the black-and-white system originally exactly matched the nominal 60 Hz frequency of alternating current power used in the United States. Matching the field refresh rate to the power source avoided intermodulation (also called beating), which produces rolling bars on the screen. Synchronization of the refresh rate to the power incidentally helped kinescope cameras record early live television broadcasts, as it was very simple to synchronize a film camera to capture one frame of video on each film frame by using the alternating current frequency to set the speed of the synchronous AC motor-drive camera. When color was added to the system, the refresh frequency was shifted slightly downward by 0.1% to approximately 59.94 Hz to eliminate stationary dot patterns in the difference frequency between the sound and color carriers, as explained below in 'Color encoding'. By the time the frame rate changed to accommodate color, it was nearly as easy to trigger the camera shutter from the video signal itself."
From: https://en.wikipedia.org/wiki/NTSC