I haven't heard TWI say that, its a real mystery eh? I will say this much - I believe it was originally introduced because any map was automatically perkable, and then there came out level farming maps. So originally the whitelisting process was just to quickly make sure it wasn't a levelling map, then they give it the thumbs up. My belief is that at the time they did not worry about things like "is there a bottleneck or pathing issue that makes this map super easy". They may have had some philosophy like "each to his own".
However, as time has gone on and those really awful but easy maps became super popular, TWI started jumping awake in the middle of the night screaming "NO NO NO textures from 1994!".They sat around and said they really hate the way lame custom maps have murdered their game and its hardcore player base. So, and this is the only part I'm not %100 sure about, about 2 TWI staff decided to take some responsibility for getting crap maps off the list. Only problem is, their decision comes down to how they feel on the day, with no written or publicly available criteria.