It is a very difficult rule to enforce and must be enforced at a Admins discretion on what is appropriate for a kick/ban. If you are caught behind the lines as a flag is captured, that is a difficult scenario and would not be appropriate to act.
But you can't always tell, and no one wants to type and ask, and you're not guaranteed to get the truth, nor will it appease everyone anyways.
As an Admin I say remove. Too difficult to enforce. And this is also Battlefield. Nuff said. It's Battlefield. Attacking an uncap can slow an advance down and distract the enemy team. They can't recap, so if they choose to waste their time in a back area, by all means, let that player have their type of fun.
You can't steal enemy tanks in this game, so, damage done by players in back areas is minimal to that in BF42 for an example.
Warfare is not a gentlemen's game.
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I've also mentioned to "the powers that be" to act to remove the 'do not attack main via Planes unless at fixed defensive positions only" rule.
Air attacks should be allowed at enemy main bases, indiscriminately, because the maps usually contain several AA defensive weapons and teams should be guarding their main bases, and generally both teams will have planes if there are planes at all.
Furthermore it is too difficult to admin some type of attacks versus, none at all, or indiscriminate attacks.
All or nothing, I say. And if it's one or the other, I say allow.