That's the community though. It's not part of Sport mode or the game in general. And gaming communities have all sorts of weird "rules" that aren't really rules but are just things that some people, usually players at the lower end of the skill spectrum, expect to be followed.
Given that GT7 doesn't have any sort of team play, if people want to try and play with their friends then they're going to be in with all the single player folks. They can choose how hard they want to race their friends, and that's completely legit. If they want to work together, the game allows it so it's a legitimate strategy.
You'll notice in game that some players will choose to work together with other players that they don't even know. See Super GT here talking about how early in the race he's looking to work with other players to try and pull away from the pack.
That's just being smart. If there's someone else in the race that you know, why wouldn't you work together to get both of you a better result? If you're playing to win then the point is to get the best result possible by any means allowed to you.
You may find
this article written about the fighting game Street Fighter interesting. It addresses players who artificially limit their success by trying to adhere to rules that make it harder for them to win.
The whole book is actually a good read for anyone wanting to take competitive gaming seriously, although a lot of the later material is more focused on more traditional 1v1 competitive games like Street Fighter, Magic: The Gathering or Starcraft and you'll need to work a bit to adapt it to concepts relevant to Gran Turismo. Still, the beginner and intermediate material is applicable to all games, and really anyone trying to be competitive in a field.