I dunno, maybe that area/district considers use of the book an "outside affiliation." That doesn't seem to me to be a correct interpretation, though, unless they are using the book as their primary text. Al-Anon sort of grew out of AA (begun when Lois and other wives began their OWN groups to support each other), so it doesn't seem that far off the beam to use portions of the text from time to time.
When I was still married to (but separated from) my second alcoholic husband, my favorite group was a mixed AA/Al-Anon-ish group, that did not list itself as either because it did not HAVE a single primary purpose. It was enormously helpful though. The format alternated between AA speakers/readings and Al-Anon (alternating weeks) and I thought it was terrific for people in both programs to get a better understanding of the "other side." I wasn't qualified for AA at the time, but I can see how helpful it would have been to AAs as well. There were a lot of couples that went, but many others who were there on their own.