The only time a mob raid results in no combat is when no one chooses to defend. Rarely is there a raid that can't be reached from some base before the field is taken. If I'm too busy, meh, I must be having fun doing something else.
Players are having fun running these. Some may want to avoid combat or really want to win the war, some may want to poke a stick in someones eye to try and start up a furball, and some might be busy eating a burrito so some mindless base hoarding fits for the next 15 minutes.
And the reason many don't defend is because it is frustrating and discouraging. You can only up so many times before being ganged by 5 and 6 to 1 odds wears you down, wheres the fun in that? And upping from another field???? LOL!!! right! If the horde is half decent at the attack (they don't even have to be all that good really) they are landing at there newly captured base before you can get there. That happen that quick.
It's not poor behavior at all, attacking bases is part of the game whether you like it or not. Just because you can't be bothered to defend a base doesn't mean the other sides should be penalized for attacking a base.
ack-ack
maybe poor behavior isn't the right term. How about poor game play?
As a "war" and battle tactics it is a great plan. Get in fast do the most damage with the least casualties and get the job done BEFORE the enemy knows what hit them.
As a game only one side is allowed to have fun as the defenders are over run with the same simple but effective plan time after time after time. How can they have fun? I know, lets build our own horde and go grab undefended bases ourselves! And so we end up with the game play we have now.
In AW, the Mafia would go head to head with CAF, or the Muskateers, and many other squads. Fights over a field could last all night. Thats the kind of game play we are missing. Too many people worrying about score, or base counts, or just plain FAILING at their missions to try it.
Keep it simple, and bring a lot of bombs

ya that sounds like a lot of fun.