this kind of behavior is just a natural reaction to siege being poorly maintained. fix the door problem, get rid of the shit unbalanced maps...clans would be alot less inclined to 'abstain' if they werent on the team they wanted.
If the bugs were fixed and well balanced maps were used, clans wouldnt care what team you put them on if you put them ALL together.
Show server logs of the win rate for attack/defense on the maps, the vast majority arent anywhere remotely close to 50/50 like a good game would have (SC2, TF2, CounterStrike)