You have drivers going on an entirely different track layout. It is farcical.
And yes, it ruins peoples fun, and not by choice. During a race, you could be near someone, and then get to the final sector at Catalunya and that nearby driver takes an entirely different track layout, gaining 2 seconds per lap, and hence you are no longer racing with that nearby competitor. It is farcical for you to suggest that we should be doing the same exploit because the game allows it.
By your same logic, it is completely okay to also hit other people and barge others out of the way, if no in-game penalty is given, because if the game allows it, then by your logic, people are choosing to have their fun ruined by not taking a farcical route on a different layout.
The same happens at Fuji, where there is an exploit involving going ludicrously far wide and staying off-track for about 10 seconds, gaining time due to having extra speed.
And surely you know about the 2 Willow Springs exploits? At both Streets and Horse Thief? Where a driver can gain up to 15 seconds on both a qualifying lap and also the final race lap. This isn't something everybody should do just because the game allows it to exist.
With so many exploits existing, it makes complete sense for people to feel unhappy with the current state of Sport Mode and decide not to play.