I think it matters where that line is located and what it is for, non-US countries seem to have 2 ways to go about it - be the line cutter or be the person that lets it happen. Not really a problem I encounter in US lines, someone always seems to say something, and I have run into more confrontational people outside the US when saying something than within.
In the domestic terminal of AKL, there was a single extra slow/crowded/busy security line, Im #2 when these 2 guys from about 10-15 back push their way to the front and no one said anything to them. I called them out and told them "there is a line" which they seemed taken aback that anyone said anything.
They then looked at the guy in front of me who they cut off, and who seemed resigned to letting them cut, but who then looked at me (for courage or for me to answer for him I don't know). He finally repeated "there is a line" (in his kiwi accent) and pointed back towards it. The 2 guys began to protest that they have to get to their flights, when finally one person behind me said they did too. Then others joined in that we all have flights to catch (everyone was running late and security had already asked for people in line w/ flights leaving in x minutes, they were not in that group). The guys moved back to their place. Everyone they pushed through was obviously annoyed, but, too polite to say anything?
Boarding groups dont seem like a line that matters much since it is hurry up and wait, the plane is not leaving while you are in line - but letting people keep cutting in security or atm or ticket lines means trading/giving away your time to them for free.