FFA or 2v2v2....

I want to get a feel for the general consensus on etiquette (if it exists) for FFA/2v2v2..etc games.

Most FFA/team plurality games I have played have been really great: weakest links are eliminated somewhat early and stronger players boom to an incredible level... however I have noticed a trend where played who are playing particularly well end up being punished for it.
For exampled:

In a 2v2v2 game, my teammate and I boomed and were taking out another team (t2)--it was a very solid battle and all sides played very well, but in the end we were clearly set to overtake them--however another team (t3) was sitting idly by , and ended up helping t2 by not attacking their units, and even sheltering them within their walls in order to help them withstand our attack.

I don't understand it. To me, this seems like bad etiquette. I would understand if t3 were to attack one of us to play an opportunistic angle to eventually win the game; but the fact that they went out of their way to deliberately aid the underdog seems in bad faith to me. Whether or not we were on track to win the game is beside the point.

What do you all think? Are there unspoken rules of play in these kinds of games, or do we throw it all out the window? Personally, I am tired of letting one player (or team) play kingmaker in ffa style games.