Half the distance instead of specified Foul

[BoomerSooner]

To answer the original question of why it is enforced this way, the simple answer is that is what the rules dictate as provided in a couple of responses. If the OP was looking for the deeper "why", I believe the rule was initially designed to solve the problem of a penalty enforcement that would place the ball beyond the goal line. For example without the half the distance rule, an offensive holding penalty enforced from the offense's 9 yard line would result in the ball being place 1 yard into the endzone. Conversely, a roughing the passer penalty enforced from the defense's 10 yard line would move the ball 5 yards into the endzone. The logically conclusion to either of these situations would be to award the appropriate score (a safety in the first situation and a TD in the second), so the purpose of the rule is to avoid awarding scores awarded by penalty.