Half the distance instead of specified Foul
 

(NFL) Just a spectator question. I have seen many times where the offense is deep in their own territory & is called with a penalty. The penalty enforced is only half the distance instead of the yardage specified by rule. Ex-Lets say a 10 yard type (holding), and the scrimmage is on the 15. Why do they not mark the ball on the 5, but instead only penalize 1/2 the distance ?

I believe that all 3 major rule codes specify that no foul shall take the ball more than half the distance to the goal.

There's an exception in NCAA -- DPI.

Unless the ball is inside the 17 yard line and the spot of the foul is more than 15 yards beyond the LOS then the ball is spotted at the 2 yard line. EXCEPT on a PAT. A DPI on a PAT results in half the distance being enforced.

NFL: By rule, when a penalty enforcement would put the ball closer than half the distance to the goal, the ball penalty is enforced half the distance.

For a 15 yard penalty, inside either 30, you'll go half the distance.
For a 10 yard penalty, inside either 20, you'll go half the distance
For a 5 yard penalty, inside either 10, you'll go half the distance.

There are exceptions to this. Defensive pass interference is a spot foul (unless in the end zone...then ball at the 1). Also, intentional grounding, where the throw is within 10 yards of the line of scrimmage...would be enforced half the distance if applicable...if the throw is more than 10 yards from the LOS, it's enforced from the spot of the foul

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.

If that were the case there'd be more appropriate ways to do it. One simple way would've just been to make it that no penalty carries the ball closer than X to the goal line, which is how they do it in rugby.

No, the half-the-distance provision was adopted to reflect the fact that a distance penalty disproportionately affects play near a goal line. It may be a bit inequitable in that regard when it comes to penalties that move the ball away from close to a goal line.

What apparently they haven't figured out (because so rare, I guess) is a similar way to handle the option team R has of taking the ball a given distance beyond the last spot for a free kick out of bounds, when that's greater than the distance to the goal line. I'm told here that since that's not penalty yardage, half -the-distance doesn't apply, leaving the implication that it's either a touchback or the option is off the table.

The Canadian formula is a little more complicated in its treatment of 25 yard penalties near a goal line, and due to the fact that the ball is not allowed to be scrimmaged inside either 1 yard line.