In amateur and school sports where there are huge disparities in talent between teams, I can see the point of a mercy rule and taking it easy on the opposition. If one team has 8 commits to Tier 1 colleges and the other is just full of duffers, then running up scores of 100-0 or whatever don't really help anyone.
But in professional sports I think it's a different story. If you're getting paid to play sports, or if you are playing another team of the same caliber/bracket as you then pile it on by all means. It's not the job of the winning team to take it easy just because the losing team decided to stop playing.
I'd be curious to hear what some of our foreign members (@gouki88, @Kiorrik, @Kernewek, etc) think about this. When I lived in Australia briefly 20 years ago I took up amateur rugby to learn it and just for the fun of it (yes it was fun until you got stuck on the ground with the ball in your hands) and we played teams that were clearly much better than us and they piled up the scores on us bigtime (them Aussies are really mean
). Nobody seemed to think much of it. I feel like it's just here in the US that we don't like "running up the score".