Well I won't say that your doubt is misplaced, but just assuming that we will only betray is also wrong.
From our perspective -
If we ally, you will win irrespective of your choice.
If we betray, then you won't win but if you also betray your points don't reduce.
So there is a very high chance that you will betray.
So in the end, both the parties are safer if they choose betray.
I'll be truthful, if you were the only one winning then I would have definitely betrayed but since there are many people winning and most likely a winner will be there in the next round, we may as well let you win.
I'll just say that I am a bit confused whether to betray/ally.