I have come across an interesting observation when using Amazon streaming. It seems that whatever non-4k I choose it is being streamed as 1080p24 (23.98). I selected the SD TV series "The Twilight Zone," which was surely shot at 30fps, and it was streamed as 1080p24. Same for the TV series "The Rifleman." British shows which seem to generally be 1080p25 or 1080p50 were sent as 1080p24.
I am using a Tivo Roamio OTA, which has the rare ability to stream at "source direct" and a Lumagen XS, which has an info screen that shows incoming and outgoing resolution. The Tivo sends OTA channels ABC and Fox at 720p and the other majors at 1080i60. PBS has a different rate which I forgot to write down.
Depending on the source, YouTube can be streamed as 23.98, 24, 25, 29.97, 30, 50, 59.9 and 60 (!). Therefore, I believe that the Tivo is not altering the steaming signal that it receives from Amazon.
But why would Amazon send everything out at 1080p24? From my amateur reading converting 30fps (old TV shows) to 24fps is a difficult process, as 6 frames have to removed. It is not a desirable conversion to make.