» Mon Mar 21, 2011 10:36 pm
Probably because if an operative shoots a mine, he looses whatever disguise he had at the current moment. While disguised, he can go do reconnaissance near an enemy objective, and spot mines without compromising his disguise. Now he can get out of there, and form up with his team as they prepare to storm the objective area, all his teammates now having knowledge of where not to set foot.
Also, as mentioned above me, it costs precious ammo. From the looks of it, your allotment of ammo without a soldier buffing your capacity isn't too impressive.