There are indeed no tripods, but when laying down there is the ground you can support your weapon with. If you ever tried laying down shooting with an assault rifle, you would agree that there is too much fatigue after only have "aimed" for 4 seconds. The crosshair sways close to 4-5 meters from a mere 100 yards distance.

I agree that it shouldn't be too easy, however I do enjoy realism first, and the call of duty elements afterwards, and this fatigue when down, is definitely one of them.