Saying hello is the first rule of our alliance, it is in the alliance description. If you can't read and follow it, goodbye.
Communication is key to war, and most of the point of having people in our alliance outside of war as well - people to discuss the game with, or maybe things beyond the game at times. Those who don't say hello when they join mostly don't change, and it isn't worth wasting space to see if they do.
If you get booted, all you have to do to rejoin is wait 24 hours and then join politely the second time.