I have had this happen a couple times while trying to snipe. Game starts, run to one corner of the map to snipe as they come out of the spawn and a minute or two later someone comes along and plops down right in front of me pinning me in the corner. I can't get out from behind him to get a new spot and even though I was there first I guess this was "his/her" spot. I don't stay there long but it is usually a good place for me to pick up a kill or two. It is really annoying but I wouldn't consider it an exploit.
I usually don't mind the campers because I will try to use them to an advantage. Have them watch my back while I snipe the other way or know where they are so I know where the enemy isn't. I try to find the best way to use the maps and if I am familiar with a player I will try to adjust so as a team do better.