they bought National Geo Societies tax free status, engineered long ago by Melvin Bell Grosvener
On top of possessing a good product in an obviously desirable part of the publishing market, the National Geographic Society also had a handful of trump cards up its sleeves. For the Society wasn't run according to the rules followed by most of its fellow successful corporations and all of its competition the commercial book and magazine publishers, atlas and globe makers, record producers and the like because the National Geographic Society was deemed a "nonprofit" organization under the rules of the Internal Revenue Service.
By the end of Melville Bell Grosvenor's 10-year tenure in 1967, the Society was selling magazines, advertising space, newsletters, maps, globes, atlases, books and records to the tune of about $500 million a year. Membership already up to 5.5 million under Melville was still swiftly growing. This membership was enough to give the National Geographic Magazine the third-largest circulation in the United States, trailing only Reader's Digest and TV Guide. As membership grew, so did the staff that the Society found necessary to service members and sell them products
https://www.washingtonpost.com/archive/business/1987/03/08/geographic-travels-tax-free/e73418fd-f46a-4ed9-9840-1af382237aa9/
https://www.nytimes.com/1982/04/24/obituaries/melville-b-grosvenor-dies-at-80-led-national-geographic-society.html