At the end of the day, SSL seats are advertised as unreserved seats first come first serve, so if someone wants to camp out all day, there really isn't much Amtrak or anyone else can do it. Don't forget this could also be a function of coach traveler (s) whose seats are in an undesirable section of the coaches (next to a screaming baby or some other loud passenger), or a single traveler forced to sit with an uncomfortable stranger. I think Amtrak did a dis-service putting tables on the upper level (this was originally intended for some kind of quick food service that never got off the ground). It took away many of the "swivel" seats that were used for individuals and pairs; in fact I never had a problem getting SSL seats until after these tables were built.
With regards to the saving of seats, as stated above you have every right to occupy an unused seat, unless that seat is truly occupied by someone using the rest room or going downstairs to get a snack. However, this backfired a bit on a trip I took on the Starlight in 2013. I saw an unoccupied seat with laptop computer in it, and asked the guy next to me if it was being saved. He said, "no, the people haven't been back in over 20 minutes". So, I sat down for about an hour watching the Starlight pass through Cascade summit, but I thought it strange someone would leave a laptop unattended. Eventually, two people DID come back and advised me they were travelling together but there were no pairs left together in the coaches, so the conductor said they could sit there until a pair opened up. They were in the diner having breakfast, which is why the seats were vacant. At that point, I promptly moved and felt bad for moving their stuff...