People in real life can easily see through grass when hiding because they have 2 eyes. Inside ArmA II they can't because technically everyone is a cyclops. With 2 eyes that are at slightly different distances, one eye can see what the other can't.
This picture is a good example if you have viewed stereo photography before:
The only game that has kind of compensated for it is Battlefield Vietnam where grass near player becomes transparent. It would be much more realistic for people going prone to see through the grass, they'd be more like ambushers instead of sitting ducks.