Some maps that have tank town, make these fields unable to be captured, hordes can swarm other bases, but those looking for a fight can always find one.
The fields that have one Vbase both change so they cannot be captured, ones with more then one vbase - only the vbase closest to the field and the AF becomes uncapturable.
Allows those who want to fight a chance to fight, and the horde can simply pick a few other bases rather then ruin it for rest of us.