They were independent contractors hired by Comcast with a Contract requirement that they badge their trucks and wear Comcast shirts. Comcast supplies the materials, there is an advantage to labeled conduit in that people digging utility test holes can easily identify the owner.
Maybe so, but if so, they're playing a very dangerous game. The legal term that comes to mind here is "agency by estoppel." Briefly put, that term means that if a company authorizes you to act on their behalf, and if they allow you to look and act like an agent of a company, then the company can be held liable for your actions.
As long as Comcast's name is on those trucks, if they screw up, Comcast is almost guaranteed to be held liable in court, regardless of whether the workers are employees or independent contractors. That legal risk is the reason that most contracts these days contain clauses that forbid you from representing yourself as being a partner of or an agent of that company.
The only communication utility that has direct buried cables (no conduit) that I'm even aware of is very old installations of telephone wires. I have run into some of the older fiber optic cables that were not in conduits but they were in armored cables with flowable fill. Such cables aren't used for anything that's not very very important. Anything installed within about the last 30 years when cheap PVC conduit became cheap is now in conduits.
Admittedly, I've only seen cables being buried for cable companies in rural areas, but they were A. coax, and B. not in any sort of conduit whatsoever. That was only a few years ago, and I doubt that practice has changed much except in areas that have gone to fiber. Mind you, that practice does vary widely from place to place, so if you live in a city (or even within twenty or thirty miles of a large city), I can understand why you would not have seen it. That doesn't mean it isn't common practice in truly rural areas.
So rural is easier, but then it's about the same cost?
It's easier, but the distance is also longer. The cost is higher in rural areas, because fewer houses can be served by a single line or set of lines. However, it isn't as much higher as the distance implies, because you don't have to bore under a driveway or sidewalk every fifty feet (and/or dig up and re-build sidewalks and driveways). Building the infrastructure while you're putting in a neighborhood is much cheaper than building it later for the same reason. The less crap you have to work around, the less it costs to put lines in. That statement is amazingly straightforward, and I would challenge you do prove it wrong.
You might find this hard to believe because you don't know what you are talking about but the cost to install the cable to this one house could be a million dollars. He could be on the outer limit of the amplification limits such that it would require them to install an entire fiber hut and amplification system. He could be on the other side of a protected refuge or there could be major utilities between him and the closest connection. In fact there could hundreds of reasons that only Comcast knows about why they can't afford to service that house. There is little point is speculating about what those reasons are unless you want to pay the $5K it would cost for an engineering and locate study to check the feasibility of the installation.
I'm not speculating. The person in question did the installation. There were no boosters, no multi-million-dollar fiber huts. The person paid to have someone trench and run a cable. The cable company lit the cable. End of story. Therefore, I do know that none of those things were necessary, and none of the things you're talking about are even slightly relevant in this case. Clearly the cost was not a million dollars. In fact, it was about $3,000. It is safe to say that if it were going to cost Comcast a million bucks to light up the cable, they would not have agreed to do so in exchange for the original poster spending a mere $3k to run the cable him/herself.
The facts seem pretty clear here, and the fact is that it cost $3k for a random individual to hire a contractor, who in turn got all the necessary permits and permissions and ran the line. Therefore in the absolute worst case, it would have cost Comcast... $3k to run the line. And that's naïvely assuming that they don't get better deals from the contractors by buying in bulk (both in terms of the materials costs and the labor costs). That's also assuming that the cable company doesn't already have agreements in place that make it easier for them to access the right-of-way than a contractor working on behalf of some random landowner. And so on.