Its granular because you can setup "VIP" or any other group that has no ads (or an admin/mod group).

It'd take a database query to achieve a mass permission copy; while unsupported as it's accessing the database directly one would:

Set "Must View Ad Island" for ALL groups:
SQL Query
UPDATE `ubbt_FORUM_PERMISSIONS` SET `AD_ISLAND` = '1';

Set the Ad Island for EVERY forum to Island #1:
SQL Query
UPDATE `ubbt_FORUMS` SET `FORUM_ISLAND_INSERT` = '1';

Forum permissions are cached in the ubbt_CACHED_PERMISSIONS table, so rebuilding forum permissions or submitting an edit should push the changes out after a query is run manually.