Why does FMS sets SAR to .mp4 to force 4:3 aspect ratio?

We record .mp4 file from live video stream from Digital Rapids and then process it with ffmpeg.

Live stream has 576x324 resolution (16:9 ratio is assumed).

It is recorded and seems fine, but has SAR value 3:4, which results in DAR equal 4:3.

It is taken into consideration by ffmpeg later on, and we get an oblate picture. That's a problem.

We came to a conclusion that SAR is set based on formula (4:3)/(width/height), so every recorded video has DAR equals 4:3.

Why is that happening? What can we do to have DAR equal to 16:9?