I will state from my own perspective and not generalize.
As a 2P, I am usually seated in the E+, instead of F, so those folks who sometimes see people sneaking up to F from E, what I am about to say does not apply to you.

Before I became addicted to this forum, I would be pissed off if I see someone sneaking from E- to E+, regardless where I sit, E+ or E-. However, nowadays, I completely agree with what someone pointed out from another thread (what do you do when the FA moves pax from E to F), just because something positive happened to some other people does not mean something negative happened to you, or me in this case, since I am speaking for myself only. Good for them to snatch those E+ seat. The meal service, the drink service and everything stays the same! As long as they do not snatch the E+ seat that belongs to me (or the empty one next to me ), I do not really give a damn.

Maybe the universe is throwing those pax a bone that day because they've got good karma... Everyone needs a bone from the universe every now and then. I just look at it this way

(a) Those pax are obvioulsy leisure travelers or they would have been seated in E+.
(b) Good for United to sell one more set of tickets.
(c) Next time when those pax want to buy a ticket, they will think about snatching one more of those E+ seats again and they will book United.
(d) (b) + (c) imply more revenue for United.
(e) (d) implies more perks for loyal customers and elites like us.

It is a all-win situation... ^ ^ ^