A lot of times you see the top college football teams run up the score on a weaker team. Prime examples of teams who do this are the USC Trojans and the Florida Gators.
It is bad sportsmanship to pass in the fourth quarter when you're up by 30. It shows you have no respect for the other team, and in the long run, it comes back to haunt you.
One day you're going to be losing big and some team is going to get revenge on you and throw a pass to try to run up the score. Or you'll end up getting beat by some team that wasn't even supposed to come near to competing with you.
Unfortunately, most teams get rewarded for running up the score and punished for not, like when USC ran up the score against Virginia and Georgia didn't, and USC jumped over them. Then in the next week, Georgia had no choice but to run up the score against Central Michigan.
In addition, when you run up the score and leave your starters in, you risk them getting injured. For example, when Oho State was up big on Youngstown State, they left Beanie Wells in. He got hurt and hasn't played since.
Your backups also don't get the experience they need.
Also the Patriots ran up the score a few times and ended up going undefeated then losing the Super Bowl.
It all goes to show running up the score is not right and has many consequences and few rewards.