Football is a competitive sport, unless you're in a sponsored top level guild WoW PvE is not. The comparison doesn't hold up. WoW is not a sport, it's not a job, it's a form of entertainment.

Sports are competitive, you work hard to best the next guy and you fight him for the rewards. Work is also competitive, you work hard to get the job and keep the job from someone who threatens to work harder/better.

WoW is neither of these things, if Casual A gets the Helm of Phat Loot, then that doesn't have any bearing on Raider B's ability to get the same. However there are a limited number of job positions and NFL contracts, so it's a totally different situation.

As far as I can see it's like this, say you have three levels of content, Hard, Normal and Easy. Player A likes the challenge from Hard and finds the other two boring. Player B finds Hard frustrating and Easy boring, he chooses normal. Player C finds both Hard and Normal to be frustrating and finds Easy fun. Why should A, B or C get any special rewards for doing what they enjoy? A likes the challenge, B likes the balance, C likes the calm, all three pay the same, all three are doing just what they enjoy, so why have any difference in reward? Because A "works"? No, he doesn't, he's just playing the way he enjoys the same as the other two.