Because it's a sloppily researched article. The land belonged to the city. They leased it to the BSA for $1 a year. The Scouts built the building on it and later turned that over to the city. The land itself, though, always belonged to the city, and is part of the same large parcel that includes parks and public museums. As far as the "in perpetuity" thing, the original agreement includes a clause that allows the city to terminate the agreement with one year's notice.
Please post thye agreement if you have it. Thanks.