As a railroad historian, I can suggest that this is a syndrome somewhat common to commuter operations, particularly those operated by government agencies as opposed to private companies (PRR, B&O, etc.). It appears that along with government ownership comes a sense of entitlement. I still have trouble convincing people that ride the MARC Camden and Brunswick lines that the state of Maryland doesn't own the railroad outright, and if I manage to convince them, "Well, they should own it!"

I would not want to be the one to tell commuters that they couldn't eat and drink on a half-hour (or longer) commuter run, especially when it could be their only chance at breakfast or dinner, but if commuters don't start showing more respect and courtesy, it might behoove MARC and others to suggest that the privilege couldgo away, after all.....

As a former daily Penn Line rider (closer job, now take light rail - oh, and please do NOT close or reduce hours at Linthicum station - it will destroy TOD as we know it) I can tell you that a major part of the problem is lack of trashcans or knowledge of them.
Each train has one at the doors and one in the middle seats (where they switch from front to rear facing).
This is simply not enough, cars are too crowded and people can't easily move to throw out trash and if they do they lose their seat.
On the way out, the door to the trash can by the door is too small for the most part and you'll cause a bottle neck trying to push it in when exiting.
I took my trash with me (unless I was near one of these few trashcans) and I think the vast majority do. Yes, you have some idiots that do not and some people simply set it down when they are done and forget as they are in a hurry to leave.
Being a daily light rail rider now I can tell that that there is no ban on eating and drinking on trains. People do it, ticket checkers rarely say anything unless the people are doing it in a messy way. Most people take their trash, but you have plenty of idiots. Sadly, many more.
I think the reason and the difference is that MARC riders tend to ride the same train, get on the same car and sit/stand in the same spot every day for their long trip and they tend to become friends or at least social with each other. Other riders will not let you get away with leaving a mess, not if you want to sit/stand there again. Light rail riders tend to vary travel times (due to increased, although still not enough, frequency). You don't see the same people all the time, a few yes, but mostly know. Friendships don't develop, people stick to themselves. If someone leaves something you dare not speak up, you don't know them, its not your business. I've occasionally seen someone speak up, and I've seen them be ignored, told to mind their own business or threatened. Only once has it resulted in the person picking up their trash.
These are two totally different animals that can't be compared. It was the same on DC metro, although more strict, people ate and drank openly on it. If people would leave their trash, which was often (there mostly papers) no one would speak up.
Transit riders interact when the train breaks down or is overly crowded, otherwise, unless they have something in common and know it (such as travel to/from an O's or Ravens game), people will stick to themselves on transit. MARC is the one exception.

I've ridden many commuter train lines in my time (MARC, SEPTA in Pa., PATH and NJ Transit in Jersey, Metro North in NY, the MBTA commuter rail around Boston, etc.), and all of them allow food and drink. As a matter of fact, the last time I rode the Metro North from Grand Central Station, there were beer carts on the platform. For the most part, there was little or no litter on the trains. Then again, the only time I've been on SRO trains is on the MARC, so I can't really say.

As far as light rail and buses, food and drink aren't allowed - but this is never enforced. I've never had a driver or a fare checker tell someone to stop eating, and I've seen riders chuck their food remains out the window as the bus was running. It seems the drivers, generally speaking, feel that rule enforcement is more trouble than it's worth, and from what I've seen, frankly, I can't disagree. I miss the days, though, when I could read on the bus in peace.

Rode the Brunswick line Gaithersburg to WDC many years ago: I remember being shocked the first time I heard an empty bottle rolling around on the floor somewhere as the train stopped. "Slobs" is too kind: MARC riders, like too many Americans, exhibit the deadly combo of arrogance AND indifference: "I don't know what I'm doing, but I have the inalienable, Reagan- & God-given right to do more of it."