Part of the reason why Amtrak does not assign seats like airlines do is that no all train cars have seating the same. At least an airline you know that a 737-500 is the same as another 737-500 on the same airline. On Amtrak, they may have to substitute one car out of the 8 on the train. And the seating may be different. The seat you chose with a window may turn out not to have a window. (Yes there are some like that.)
Look these are all desperate and well meaning attempt to defend the indefensible in Amtrak.
The same airline often has multiple different seat layouts in the same type of plane, and they actually are able to keep track of which specific plane is going to be used on a flight and sell seats accordingly with specific seat assignments. It does happen sometimes that they have to substitute equipment, and when that happens they simply deal with it on a case by case basis. Very often the number of seat differences are at most a dozen or two. The rest get to keep the original seat assignment. At other times the entire flight might get canceled in which case people have to accommodated on other flights. But none of these is a good reason to not assign seats.
And on Acela, they do not turn the train at the destination, so 1/2 the seats face one direction and 1/2 face the other direction. You may discover that the seat you chose has you riding backwards, which you may not like.
Somehow I have difficulty believing that. I believe Acela seats are turnable and they are turned at the ends even though the train is not turned. but I am happy to be corrected, but I don;t recall ever seeing an Acela with equal number of seats facing forward and back, and I rode some just the other day, both in First Class first and First Class last orientation.