So what are the ethical considerations here? If this was an airline forum, everyone would immediately chime in that hidden-city ticketing (or whatever you would call the variation here) is not practical since the airline would (a) cancel your return ticket, or (b) you might get re-routed in an IRROPS situation meaning the airline would send you to your ticketed destination, not your intended destination, or (c) the airline would eventually figure it out if you did it too often and penalize you financially or by docking FF miles.
Those aren't really concerns with Amtrak. For (a), you can easily avoid this by purchasing a one-way ticket, not to mention the fact that Amtrak doesn't really have a way of tracking where you get off the train. For (b), the chances of truly getting re-routed on Amtrak are virtually zero. And (c)...well, the answer to (a) basically applies here too. They can't penalize what they don't know about.
So from a practical standpoint, there is essentially nothing to worry about by purchasing a ticket from Point A to Point C and getting off the train at intermediate Point B. (Unless you are checking a bag, which is a complication easily avoided for most passengers, and isn't even possible for many destinations.)
But what are the ethical implications? By doing this, you are knowingly purchasing product "A" and using product "B". You are not only denying Amtrak the extra revenue they would have received had you purchased the correct ticket, but you are also denying them the revenue they (potentially) could receive by selling a fare to a different passenger who would have boarded at the intermediate point, in a sold-out scenario (applicable, since we're talking about busy travel periods). In fact, if you were denied the chance to buy a ticket because someone was "occupying" a seat but had actually de-boarded, how would you feel?