The answer is simple: because it makes things easier. Similar to using cheats in singleplayer games, modifying certain elements in the favour of your gameplay will makes things easier then they would be with normal circumstances.
I don't know about race, but with 0.5.0 laserjumps became very popular for instagib. Even so popular that it was hard to find a server without it. The 0.4.3 instagib core couldn't really understand what the fun was about, after all it made movement and aiming much more randomized and destroyed the main point of using the hook to gain speed and perform skilled movements.
It took some time but after much discussion we discovered that is was precisely because it removed these advanced elements from the game that 95% of the servers used this function. All of a sudden people didn't have to learn how to hook properly, because they could now accelerate with just one shot and jump as if there was no limit on double jumps. This turned a clean genre into and oversimplified and therefor pretty boring genre when you had the experience of the 0.4.3 core. It removed the need for one of the several main aspects of instagib, that was, movements.
Good thing more and more servers are coming back from this, making more and more clean instagib servers pop up every day. Because even experienced laserjumpers can be beaten by using the experiences gained from using the hook these players realized that they had to take their gameplay to the next level, which involved the mastering of one of Teeworlds most original concepts, the hook.
It's probably the same thing with race. Disabling gravity or adding max hook length makes things a lot easier for new players, despite ruining the point of the mod as it was envisioned by the original players. Let's hope that people will see the light here as well and see that making any game easier then it's supposed to be will only degrade the level of fun involved in beating the game as it should be.
