It wasnt Me, but heck it sure could have been.
If the OP had purchased all 3 seats and had BPs for all 3 then the OP would have been entitled to All 3. Barring that if Im in a bad seat (non-aisle) and I see an open aisle seat , you better believe that Im taking that seat. Now had the OP presented me with their BP showing me that in fact that seat is theirs then I probably would have asked OK, are you sitting in it if not I am. If they say yes, then I probably would have gone for the window seat assuming I had a middle seat.
Now if I returned to my non-aisle seat and saw that the OP wasnt actually sitting in that aisle seat then I would have moved back into it, even thou they had a BP for it since they obviously werent gonna be sitting in it and if I didnt care if the person in front was gonna recline its mine .
As for them having taken over the middle seat, nothing wrong with that, the OP should have simply beaten them to it. Or lifted the arm rest or something. Ive been there before and well all I will say it didnt take long for the other person to get up and leave cause they werent able to relax with me going an dcoming so often and unfortunately always bumping into them as I did so. thats only after they had taken over the other 3 seats (it was a 747 in the middle section), As much as I wanted all 4 for myself I was able to see it was gonna be a 2 & 2 deal but I wasnt gonna have the 3 & 1 deal especially when their BP wasnt even for the row. But I never thought or saw the whole row as belonging to me. It was my loss on the 4 since I usually would have spread my things out over the other seats, I didnt so I had to live with 2 & 2 , but no way a 3 & 1 deal with me with the 1 seat. Had the guy been decent I would have lived with the 2 & 2, once he basically told me where I can go, that was that I was getting rid of him, and I did. But that guy had a right to sit on the row.