If an attack breaks the enemy lines, and forms a sailent, they are behind enemy lines, but only in the sense that they have gotten behind the main enemy defense line. Bit they're not IN enemy territory, because the attack dislodged the enemy from that particular possition.
And I think the spawn placment is more for gameplay than because HTC wanted to represent being behind enemy lines. They're close enough that you don't have to drive 40 minutes to get to a fight, but they're far enough that the defenders have a fair chance to scrable and up tanks or aircraft.
Lusche is right though, TECHNICALLY, the ground is either friendly or hostile. But its just that, a technicallity.