His reasoning is safety, because it's supposedly (and I guess understandably...) unsafe for cars to be moving at very different speeds next to each other. I assume what people are afraid of (or what actually happens) is that people in the lane moving slower might drift/shift into the lane moving faster and cause a collision. When this happens while both lanes are moving at similar speeds both cars are more capable of avoiding it.

I've been there. I've been in the backed up lane in traffic with the lane next to me moving much faster, near the speed limit. I often try to get into the fast moving lane and it is a bit scary because it can be difficult to see far back enough to get into the lane without cutting somebody off. I still believe it is the fault of the people in the slower lane for being there though, and it is their responsibility to stay in that lane or find a safe opening to get to the next lane. If I see a bunch of stubborn early mergers, I will assume that is where they want to be and I will drive past them like a bunch the bunch of parked cars they are. But I will indeed not necessarily drive 80+ mph and I will pay close attention to the movements of the cars.

There are three reasons to drive slowly in the right lane.

(1) It's safer (matching speeds reduces odds of and damage from collisions)

(2) It's more efficient (If you zoom ahead, there is a nonzero chance that drivers in the left lane will be annoyed and attempt to block you, reducing global efficiency. Note that there's no loss in going slow since the bottleneck is the constraint. As a result, going slow can only gain and never cost.)

(3) It's fairest (people who arrive at the same time will leave at the same time, regardless of their lane)

Another comment lays out my logic further: https://news.ycombinator.com/item?id=8084137

I know it feels silly to drive slowly in the right lane, but it's safer, more efficient, and fairest. Why wouldn't you do it?