It's not just a matter of meals in the dining car that led to the restriction of two adults per room. Meals are part of the equation, but again are not the sole reason.

Amtrak has simply had too many complaints that the room was smaller than the people expected it was. Amtrak tried for a while only allowing people to book 3 into the room, by forcing you to speak with an agent. And the agent was required to tell you that Amtrak does not recommend doing this as the space is limited. People would say, "ok, I understand. No problem." Then turn around and complain that they didn't fully understand that it was that small.

And of course some agents would claim that they said that and even note it in the PNR, when they never bothered to say anything at all.

So it was just easier to deny booking 3 into a room than play that game.