You may have missed the part where OP said he went into a vacant row.

Let's take it one step further...does everyone here agree that putting your stuff into a vacant middle seat when you're in the window seat is theft? Because by most of the logic displayed here, yep, that's theft too. To me, the people in E+ who would be mad about this are playing their DYKWIA card more than anything.

Nope, didn't miss a thing. Someone takes something that is more desirable, has a price tag because of that, and someone wants that thing without paying for it. Not cool.
Now if the airline decides to give me back the money I paid for the privilege, I would be totally cool with it.

and to go one up: what if there's an empty row in BE? Is it ok to just take that seat too then? Or is that really a magic curtain between Y an J?

Oh boy the can of worms.

If you take anything from this post OP.

Some of us don't think you did anything to warrant being called a thief.

And some of us thinks you should be ashamed of yourself.

What about the seat selection process, then? Many airlines charge extra for seats, even in economy, such as BA. If you paid less for a middle seat, then luckily the window or exit row was open, and you shuffled over, this is the same for you? Right now on LAX-LHR I'm given the option of paying $30 or somesuch to buy an exit row, and $60 to get a window, and even more for an exit row. Would you get upset if someone didn't pay for choosing a seat, then decided to move to the window in a mostly-empty row?

Because I don't think you'd willfully sit in the middle seat if the window was open. Etc etc. You can get so pedantic about this, that it just isn't worth giving a second thought to.

They do suffer a loss actually: the perception that it is worth $$ to buy up to E+. If people just go sit there without paying, other people that do pay will not do that again, and just take it for free next time too.

So move up to the free BE seat next time you fly. See how well that goes for you.

The problem is that the cocky/arrogant/entitled will rule the plane. Just because you are arrogant enough to claim something that is not yours, does not make it acceptable.
Now of course you could ask the FA, and definitely in the case of a window for a middle on your own row, there will probably be no objection.

(Shrug) theft of service.

my god, what a bunch of puritans. OP did absolutely nothing wrong.

What are you doing?!? Quick retreat before they call you arrogant and entitled. Learn from my mistakes my dear, nothing works.

Just so I am clear about your position: You board a flight. The flight attendant points out that there are some nicer seats available but you may not move to them unless you pay. You're told to contact an FA to pay them so you can move. So instead you wait until their backs are turned and sneak up to the better seat.

"Absolutely nothing wrong" with that? Is that what you're saying?

No I haven't and if I had paid for something I would be annoyed that someone helped themselves to something they were not entitled to.

Don't forget anarchist, the rules dont apply to you obviously.

Nope

I doubt anyone can dig up a single incident of anyone being charged with a crime for this. If the airline caught him and asked him to move and he refused, then yes, he could be charged with a crime. But assuming that he complied and went back to his seat upon request, it would be difficult to prove all the elements of theft of service. This is, of course, that he had paid for a different seat on the flight. If he had sneaked onto the plane, it's a completely different situation (analogous to sneaking into a movie theater).

Those of you who consider switching to E+ to be theft, please answer this question. If I am in a middle seat and the aisle seat next to me is open, can I switch into it? Or is that theft of service as well? (Assume that the airline charges more for aisle seats, which many do these days.)