It doesn't, as you say, mean that someone on the edge of a graph commutes all the way to center---after all the drive time for some of these routes is more than 8 hours.
You could argue that choosing 50 clusters is arbitrary and it would make more sense to severe connections below some threshold "quality" and see how many clusters meet that criteria.
I live in SF, and at work my team consists of ~16 people. Of those 16 people only 3 live in SF (city/county). The other 13 live in the East Bay or south of the city. Of those who live outside the city the majority doesn't get paid enough to live within city limits; the others get paid well but have families and couldn't reasonably afford a large enough home in the city.
People are very willing to convert time into savings (on housing). And sometimes they have no other choice.
So most of the population here is within around 20 mi of the center of Portland, unlike Phoenix AZ where it sprawls out 100 mi or more. I'm guessing there are people who travel from/to nearby regions (e.g., Newberg, McMinnville, the "wine country") for commerce, tourism, and probably some commuters.
Perhaps there are similar considerations in other regions like SF which certainly has "tentacles" that stretch a long way out there.
May be, but they aren't commuting from Chico! That's a 3 hour drive in good traffic.
edit: Original post said Sac, not Chico...
You have to drive 200 miles out of the way to get from one side of the Grand Canyon to the other. Since the northern Strip is very sparsely populated and relatively poor, the number of people there who have the means to regularly commute by plane or helicopter must be zero.
The region north of the Grand Canyon should really be clustered with Utah or Nevada, not with Phoenix and Tucson.
Use the number of commuters as a metric for 'closeness' of two areas. Then take it further by asking "how 'close' is Portland and Ashland?" by using the sum of the 'closeness' of each little hop from area to area.
This map is showing clustering, finding groups that are nearest to each other, using that metric as a distance function. Find me 50 areas where '# of commutes' are the distance function, and you get this map.
Ashland and Portland aren't very close geographically, but in terms of economics, they're in the same neighborhood of the country.
I worked with a consultant who lived near Scranton so he could commute to a large number of places. (Syracuse, Albany, NYC, Jersey, Philly)
Drove 3 hours a day one way!
The final clustering excluded trips between points >=262 km from each other though.
Erm. No, it's not. I'm sure there are a tiny percentage of people doing this out of necessity, but spending 8 hours a day commuting is not even remotely 'reasonable'.
No one commutes from Ashland, Oregon to Portland Oregon - it's a 4+ hour drive. Yet they're in the same pink area. Meanwhile, no one really commutes from Bend to Portland either (2.5 hour drive in good weather), but we're brown over here.
There's certainly some more cultural affinity between cities in Oregon's Willamette Valley than with the more conservative central and eastern part of the state, but even that only holds for the larger cities - there are plenty of pretty 'red' places down the I-5 corridor.