Moving to the seat next to you is not theft if the seats are the same price. Moving to a seat that is more expensive is, in fact, theft.

I think you missed the example of the movie theater above. If you paid to see one movie then decided to walk across the hall and sneak into a more expensive movie isn't that a crime? It's exactly what the OP did. Remember, the FA announcements make it very clear that you have to pay extra to sit in the E+ seat.

When I described my hypothetical scenario, I said that the airline charges more for aisle and window seats.

Just to clarify, you are saying that on such an airline, if I am sitting in the middle, and I move to the empty aisle seat so that both the window passenger and I can have more room, I am committing theft?

I would say that switching cabins is analogous to moving to another seat within the same theater. I don't know of any movie theater or other venue that will object to this, even if certain seats (such as balcony) are more expensive. In fact, at some of the best concert halls in the country, they actually encourage it. If the FA said not to do it, then it might be a crime, but "I didn't hear the announcement" would likely be enough to get off the hook because most states define theft as something done knowingly.

Paying and then walking into a different theater might be analogous to using your boarding pass to get on the wrong plane. Airlines have better security than movie theaters so this would be difficult but not impossible.

In my example I referred to a more expensive movie. You buy the ticket for a regular movie on a standard screen then sneak across the hall to the iMax 3D movie, for example. Yes, they would object.

Time to end the wheel spinning, so: last post and comment. Either people are honest, or they are not. Either way, it says a lot about them.

Once I'm confident that doors are about to close, I take the aisle seat or even just the middle seat (if there is one). It cuts down on others doing what you experienced. Am I ultimately preventing someone else from doing exactly what I'm doing and from getting something that I'm not really entitled to either? Yes. And I sleep well at night. And I'm fun at parties, I think.

Hypothetical: a newbie pax boards a plane, notices lots of empty seats (he/she) is supposed to be seated in a middle seat between two sumo wrestlers; he moves to one of them (which happens to be a more expensive "special = E+" seat). Pax has committed theft of service--but didn't know it.
Same scenario, but pax intentionally took the premium seat, knowing he shouldn't be there.
In both instances a FA should inform pax of his faux pas. Now if pax doesn't return to his "originally scheduled" seat, pax could/should be prosecuted for theft of service.

I've been on some relatively empty DL domestic flights recently, and in every case the FAs have announced that Y pax can switch seats if they want -- but not into the EC rows. "Premium" Y seats seem to be fair game, but they do protect the EC section.

I totally agree! But some members here really was hell bent on making it seem like it is a bigger deal than it is. If it bothers you that someone moved from E to E+ then by all means say something. If it doesn't bother you or the FA then that's fine too.

Thought "Premium Y" is the same as EC? Or do you mean preferred seats? (guessing you're talking Delta here)

You're right -- I'm talking Delta and I meant the "Preferred" seats.

First of all: whether prosecuted as a crime or not (which I agree would be extreme), it still does not make it ok to just claim something that you did not pay for.
If you fail to follow the instructions of the FA (you need to sit in that seat, not this one), I think you will be thrown off the plane or arrested on different grounds though.

For the second part: if you would ask the FA if its ok to move from the middle to the aisle, I'm pretty sure everyone would think you would get a positive response. Now the same question but moving to an E+ seat? Very big chance you will get a "no".
I guess self-upgraders like to go by "better pray for forgiveness rather than ask for permission."

Whilst I agree absolutely with the sentiment that it shouldn't be done, "theft of service" made me laugh. Nowadays there is no service on US airlines. The only think that has been stolen is the expectation of service, and it's the airline that has stolen that expectation.

Well, only if you're cool with taking something that doesn't belong to you.

And I'm not sure why it makes one bit of difference if it was an empty E+ seat or entire row. The OP didn't pay for it and others did (or earned enough status to qualify). It's not yours to take.

Lol, no argument here!

Anarchy is just fine until you meet a bigger, meaner anarchist.

Cool with taking something that doesn't belong to me? This crime, like I said before is a matter of discretion. The difference we have is a matter of opinion. When I'm sitting in E+ and I see someone move, it does't bother me. It may bother you and you have all right to that opinion.