There's two camps of fans in the running up the score debate. There are the fans who say that it's perfectly fair and then there's the fans who call it bad sportsmanship.
This writer says that it's perfectly fair for NFL teams to run up the score.
This is the NFL. This is not kiddie rec football, where you have the kids play nice so that no one gets their feelings hurt and where participation trophies exist. Every single player is a paid professional who makes at least six figures to win the game. The offense is getting paid to score as many points as they can. The defense is getting paid to keep the opposing offense from scoring points and to try and score some of their own if they can muster that achievement. If the defense isn't able to do their job, the offense should not stop doing their job. That's unprofessional on every level.
The same argument wouldn't be made against the defense for not allowing the opponent to score any points. It wouldn't be looked upon as running up the score if a team was winning by 40 points and returned an interception for a touchdown, so why does it matter when the offense does it?
And it also isn't fair to the fans. Football fans pay hard earned money to see their teams play, and in return we want to see the best possible product on the field. Part of that is not pulling any punches. If quarterback Joe Flacco is on pace to throw eight touchdowns in a single game, or if Justin Forsett is on pace to break Adrian Petersons' single game rushing record would you want them to stop just because some people might call it unfair? No. That takes away from the fans experience and that's not giving the fans the best possible product.
Not only does it put a more entertaining product on the field for the casual fan, it means a lot to the players and coaches involved. Even without the single-game achievements for players, the effect on the season stats could mean the difference between having a 2000-yard season or a 1700-yard one. One could get you into the Hall of Fame, while the other is almost ordinary now. For coaches, the ability to beat on any NFL team so bad that it becomes "unfair" only helps when it's time for coaching vacancies to be filled. The same owner that was probably cussing up a storm at an offensive coordinator the previous season will be the first to interview him the following. When you consider that the jump from a coordinator position to a head coach is typically what most coaches aspire to and comes with a hefty pay increase, you can see why guys shouldn't care if it is frowned upon.
Running up the score is perfectly fair and teams shouldn't shy away from doing it because of some old moral code.