I see it differently, the defender is protecting the ball and seeing it out. That's why i think officials allow it to happen, and more importantly, you don't see the players kicking up a fuss about it really, unless it's injury time and they're chasing a goal. There are plenty of unwritten 'Rules' in a competitive football match, and rightly so in my opinion, because the official would end up blowing his whistle every thirty seconds if they didn't exist. Every corner would result in a penalty,every deliberate contact would end up with a free kick, thus stopping the game and making it less entertaining. I believe in more power for officials to referee the game as they see fit, and for players to bend the rules as far as they can in order to get an advantage, it's not just football that equates to, but probably all levels of high level professionalism. There's nothing wrong with getting your body between the ball and the opposing player in my opinion, and the defender has absolutely no obligation in letting the attacking player have the ball, and that's accepted on the field of play.