I maybe wrong here, but here is what I think about the races in GT2 and 3 having less laps and softer rules:
I'm at DR A. Back in january this year my wheel broke, and I started to play in another account with the dual shock, started from DR E and went up to almost DR A with the controller. During this period I have entered some races in the the exhibtion series, when they had the same rules for all leagues. More Specifically the race in Spa that took almost 1 hour to finish, the lobby started with 16 players, 1 of them quit or disconnected during qualify, 15 started the race, but only 5 cars crossed the line at the end.
I think it's hard to please everyone, and I also don't agree with the rules being different for GT1/2/3 leagues, but I also think it's ok for it to be different, what I don't like and agree is that they give no option for you to be in a long or short race, the only determintating factor being our DR.
It is not the hardest thing in the world to get to DR A, but everyone has their priorities, someone maybe give it all to get to DR A and fail, while some other person don't really care about DR, and just want to race.