Do note, none of the "cruiser" capital ships have a culture bonus, only the more expensive "elite" ones do. So if you have say, a fleet of VSDs, they aren't giving you a culture bonus, though it seems you know this. The reason this occurs is, when the AI gets a planet like any of the Tropical ones, or something else with a culture bonus like an Ecumen, they love to utterly spam HoloNet Broadcasters. Like, you can find planets with 16+ of those things on them. While you might be overwhelmed with culture at times, I have been as well, culture itself hasn't been changed for years, not even when NU was introduced, it's the planets' changes that are allowing players to pull this off.