I’d be interested in the full explanation of this too, but in the meantime I’d like to make two points
1, In the past meeting attendance was limited to bona fide BAA members, but members of affiliated societies could also attend. This may no longer be the case with live streaming. This also applied to other BAA membership benefits (e.g. advice from Section Directors).
2, One of my local astronomical societies subscribes to several astronomy magazines and it is a very real benefit of membership to be able to read current magazines purely for the much cheaper price of membership. How local societies will still be able to obtain the JBAA & Handbook is, currently, not obvious.
British Astronomical Association
Supporting amateur astronomers since 1890