That was a gesture by the Haas Family that owned the A’s. It had previously been considered joint territory. The a hope was that the Giants would stay in the Bay Area. There had been talk that they might relocate to Vancouver or Tampa. Tampa had their stadium construction started before they had a team. I think there might have also been an agreement in place to move to Tampa, but that was after the voter measure was rejected.
That’s one thing that A’s fans hate, that a kind gesture (that at least delayed the relocation of the Giants long enough to assemble an ownership group) wasn’t returned in kind.
Looked into the history again, and it was 1990. Kind of a handshake deal, but the A's claimed it was contingent on moving to Santa Clara County while the Giants claim that it was unconditional.
Late Oakland owner Walter Haas gave the Giants the OK to assume rights to San Jose in a favor of sorts to former San Francisco owner Bob Lurie when his team was considering moving to Florida. The deal basically happened with a handshake -- and "without compensation," the A's wrote -- and then was approved by baseball's owners.
The A's said that "MLB-recorded minutes clearly indicate that the Giants were granted Santa Clara, subject to relocating to the city of Santa Clara."
Said the Giants: "The Giants' territorial rights were not granted 'subject to' moving to Santa Clara County."