dunno i think its cos back players make or break the game cos its them thats covering everybodys ass and directing front players to where the opposition is and they can always push up and do a front/mid mans job if the need arrises so when /if ur back men get shot out other team members can become sitting ducks
the back player probably does have the most difficult job, watching over the fronts, relaying information, moving up and refilling positions. Everyone relys on you if you are the last in and you will get the flak if you loose.
My 2p is that each position on the field has a difficult job. Yes back have to keep up a high rate of fire, and take on the role of shouting the orders so to speak. Each position has different roles to play and so different difficulties results. I play snake and am always thankful for having a good black player helping me out. I would not shout at my back player when he is the last out, we're all on the same team. If you do loose as a result of your back being the last person in, then its not his/her fault it's the whole teams fault.