That is indeed the complaint. It’s extremely sensible to want 2 seats to yourself. It’s also sensible for Amtrak to seat passengers in the most efficient way which means assigning solo passengers together. Both make sense.
If you have 20 solo passengers in a car that seats 60, if you let them sit anywhere they want you now have 20 single seats taken up scattered throughout the car. That leaves you with only 20 seats as pairs.
If you assign seats, those 20 passengers will take up the first 20 seats in the car leaving you with 40 seats together as pairs.
Traveling on VIA this fall (3 different trains), I had assigned seats each time. I was even asked if I wanted to sit facing forward or backwards and was given the best window seat.
Seems like it can be done, if AMTRAK wanted to and cared!