I think the issue comes down to keeping AA, well, AA. Consider newcomers. People get an impression of AA based on what happens in meetings. Newcomers come to meetings and hear literature read aloud. When they leave the meeting that same literature is associated with AA. If they hear words from the 24 hour book in their first few meetings, no big deal. If they hear quotes from "Mein Kampf" that's a problem.
I believe the 'conference approved' label helps groups to stay on track and not get involved in "outside issues". Again, the 24 hour book is not going to be a problem IMO, but other literature might.
As far as other (non conference approved) literature is concerned, consider what Bill said of AA, "We are only operating a spiritual kindergarten in which people are enabled to get over drinking and find the grace to go on living to better effect. Each man's theology has to be his own quest, his own affair." (emphasis mine) "As Bill Sees It" p. 95.
What is read in a meeting and what you read for your own spiritual development are quite different situations. They should be treated accordingly.