Originally Posted by LAX
. . . Since the seat is unassigned and unsold, thus no one on that plane technically has any right to that seat, including the OP. . . . Now, can someone explain to me how other people have "more right" to that empty seat next to the OP than the OP? . . .
LAX
The crucial distinction is that the hypothetical passenger moving from their assigned seat to 12B would still be seeking to occupy one seat; if the OP vigorously attempted to prevent this passenger from sitting in 12B, she would be seeking to occupy two seats (while paying or having status for one).
Once the doors close, the reasonable expectation is for passengers to move to unoccupied, unblocked seats that they deem superior. (In their contracts of carriage, the airlines certainly view seats as fungible and refuse to obligate themselves to honor seat assignments.)
On an ERJ, 12B is a coveted seat, so the OP should not be surprised or overly upset if another passenger moves into it. Would it suck, yes; would it be unfair, no.
Whether to place personal items on the empty seat in an attempt to discourage a fellow passenger from sitting in it strikes me as a matter of personal ethics, and this thread reveals a spectrum of positions.