My point was that the attendant may move two people sitting next to each other who don't know each other to other single seats so the two people boarding could sit next to each other.
I would be
quite upset, if I arrived at the station early, and stood in line for a long time, just to "snag" a good seat (electric outlet, scenery side), only to have the attendant later move me to a bad seat, for some late-arriving couple can sit together.
Would you move your seat?
To answer your question directly, yes, I would. It's called "common courtesy", and it's still present out here in flyover country.
The last time I rode the Broadway Limited in 1990, as was my habit on that train when boarding in Greensburg, PA, I got a coach seat -- an aisle one at that, the train was crowded -- and went straight to the diner for (real) railroad french toast, orange juice, and coffee. After a pleasant breakfast lingering over several cups of coffee, I went back to my seat. My memory these days is hazy, but I think I even stayed in the diner for the ride over Horseshoe Curve.
You guessed it: someone boarding at Johnstown had claimed my seat. I pointed to my seat check, and said words to the effect of, "Excuse me, but you're in my seat."
The person reacted with incredulity. I pointed to the seat check above her head. She refused to move. I got the conductor (being quite used to such things from numerous trips on DB), who backed me up. She moved, after much grousing with the conductor.
All of which leads me to conclude Amtrak not having individual seat reservations on a designated coach -- at least by line number, DB's reservation told me the exact car assigned(!) -- is indicative of how half-a###d Amtrak is run.