I don't think it's bad sportsmanship at all in pro sports. The other side is getting paid just like you. I do think it's pretty stupid, though, especially in the more physical sports. There's no reason to risk injury to a starter if your already up 28 in the fourth quarter.
In amateur competition, it's about the most unsportsmanlike thing you can do. I remember losing a basketball game 96-4 when I was in fourth grade. It was a sixth grade parochial league and we were a very small school. The coach of the other team used the first game against us as a tryout for his A team. So not only were we going up against kids much older than us, but we were going against there best players as well.
It's one thing to make sure your starters get good playing time and to go up by a wide margin in the first half, but a decent person knows when to pull his kids back. We actually played the B team later in the playoffs and I know we lost by less than 10, so it's not like he even had to tell his kids to quit playing hard, all he had to do was put in more of the backups closer to our height.