Its not that you have to camp... Its that people will camp heavy because its the most efficient. If I miss heavy once or twice in a game that means I'm going to have to do an entire other match and try again, if you mercy the enemy team or get mercied it means you get even less chances at heavy ammo during that match. Since quite a few matches in IB end with a mercy you only get 1 box for the entire match, hence why people will camp heavy ammo to get a higher chance at securing that single box for the match. Its the disparity in the quest step where there is a chance that you don't even get any progress while every other step in the quest can gain progress 99% of the time in every match you play.