They accept modders. Modding in singleplayer is perfectly within policy. What's (understandably) not acceptable is modding things like models in multiplayer. While in a perfect world, nobody would take advantage of it, the reality is that the most popular multiplayer mods would be ones that let you see your opponents through walls, fire through walls etc. You might think that drawing a line between fun and exploitative mods wouldn't be too terribly difficult, but how much of a model change is fair? If I mod player heads to be bigger because it looks funny, that might also make it significantly easier to headshot these models with my client.
A dedicated multiplayer environment where people can load in common mods, and only play with others with the same mods is really the only fair option, and that's just not the way GTA Online is set up (it's more like an MMO instead of individualized servers as Minecraft and Garry's Mod have).