In my opinion, the issue is applying railroad thinking (rigidly fixed route) to an interstate bus situation (some flexibility).
While we railfans debate what should have happened, the interstate bus company that Amtrak competes with on this route has provided reliable (if not particularly comfortable) uninterrupted service through the area 2 or 3 times a day each direction throughout the the Carr fire.
Making it the customer's problem to find a way home (with their checked luggage in tow) is the surest way to not have that customer in the future.
Who's to say that were charter buses available for the passenger loads? And ones who were willing to run their buses and employees through a potential hazard zone?
As far as turning the train back at different stops, the Starlight is a massive (passenger) train, and to use your analogy of a bus, it can not simply be physically turned around anywhere. If there is a nearby wye or loop, fantastic! But are the crews qualified to run there? Is there servicing facilities to water the coaches, empty the toilets, most importantly fuel the engines?
There's generally a lot of things many people don't think about that have to line up in the background to make plans that seem good on paper actually work. It's not to say this was an ideal solution by any means, but there may be reasons beyond Amtrak's control that caused them to arrive at its decision. Contrary to popular belief, the company does want to get passengers to their destinations....
Good points on the turn points at smaller stations. Also looking in Google Earth, I also see those stations would have difficulty staging the buses. That's why I posed it as a question, thanks for the response.
I still don't agree on the lack of a SAC-KFS bus bridge though. I've been on way too many planned train trips that turned into Amtrak bus trips in the past, including a long planned Donner Pass Zephyr trip that turned into a SAC to Reno bus trip because the Zephyr was very late westbound, so they turned it at Reno (instead of Emeryville). Also the Pacific Northwest Coast Starlight/Cascades bus bridges seemly happen all the time anymore. Amtrak knows how to put train passengers on buses. If you can't get enough buses, then you accommodate as many people as you can, and then make alternate arrangements for the remaining passengers.
Regarding the decision on the I5 corridor, that, in my opinion, is where you rely on the professionals in that area of expertise. In this case, that would be CalTrans and CalFire, neither of which issued
any related I5 travel advisories.
The decision to evacuate an area, like many things, is based on many variables. It takes a long time to evacuate people. The small local roads that EMS/Fire needs for fire fighting support efforts are the same ones that people would evacuate on. The evacuee destination is in Redding, east of I5. Unlike Interstates, many of the local roads often have tree stands very close to the roadway edge. Most all property that abuts the I5 right-of-way doesn't have access to I5. Residents have to rely on local roads.
Regardless of all that, there are alternative SAC-KFS routes, in the highly unlikely event that there was an I5 closure. Amtrak not addressing the problem did not make the problem go away, it merely made it the customers problem to solve. The "can't do" attitude wasn't very impressive. Solutions customers have come up with, so far in this thread, have been to buy a full fare no-advance purchase plane ticket, great if you have the money and are going to a destination served by air service, or take a Greyhound bus. Those logistics can be difficult for a single traveler to arrange, particularly an elderly traveler with luggage. I'd hate to think about my elderly mother having to do that without my remote help (I'm in Alaska).
BTW, the alternate SAC-KFS road routes only take about 1 to 1-1/2 hours longer.