The speed limit is a maximum, not a minimum. There's no requirement that all traffic must drive the speed limit.
In the case of that second video, I can think of a number of reasons a driver might drive below the limit:
1.) Nighttime visibility issues.
2.) Weather-related visibility issues (the road was wet).
3.) Unfamiliarity with the area.
4.) Looking for a specific destination or street on which to turn.
as a delivery driver, i am frequently under the limit in some places due to #3 or #4, often in conjunction with #1 (poorly lit signs, houses, etc.)
myself, if i see i'm holding up traffic, i'll pull over and let people by. but i think i'd be a little rankled if this guy was flashing his lights behind me. but i also have a lit sign on top of my car, so i like to think people might cut me some slack seeing that.