Fair to say that most paying passengers (whether cash or Avios) would consider this something of an irritation in a cabin where one has expectations of a reasonable level of peace & quiet. Those who are quick to assert that these sort of freebie upgrades should be of zero concern to others (the familiar mind-your-own-business quip), and / or that one should somehow even feel pleased for the beneficiaries, might wish to re-think their perspective (but that’s probably asking a lot ..... !)

It’s a fact of life that the Captain is more or less omnipotent once the doors are closed and in almost all situations that makes total sense. However, that power should not, in itself, be accompanied by a lack of respect for ordinary, ‘non-connected’, passengers.

If a passenger were to come from another cabin to spend (excessive) time chatting to friends & family, they can at least be told by CC to return to their assigned seat. That’s obviously not possible when a Captain or FO happens to be the ‘culprit’ ; all the more reason why a professional standard of behaviour and courtesy is important.