Players outside of playable map in Rush game mode - exploit or legit?

I was playing Rush on Operation Firestorm as the defending team (Russians) and the opposing team was able to sit in the hills to the right of their spawn point and snipe. When I tried to rush that section of the map I was getting the warning message that I need to return to the map.

Update Another tactic I see players using is to camp within this dedicated zone and spam mortars.

Is this legitimate or is this an exploit? It was quite annoying and gives the opposite team an unfair advantage.

I was playing on the Xbox 360.

Solution 1:

It's actually like that so that the defending team doesn't spawn kill the attacking team. All maps have zones where one team can't enter, those zones are defined with grey stripes in the minimap