This was part of the "California Dream" Barbie doll package, when people bought the doll it came with a flexi-disc with the song on it. If you zoom in on the award you'll see the Barbie graphic on the record. Whatever kind of deal was struck between Mattel toys and the band to cover royalties or an outright payment up front to use the song, if 2 million of the California Barbie dolls were sold, Mattel gave them this award for 2 million sales of the song too. Notice the award came from Mattel toys, and not a record label like other gold/platinum record awards.
EDIT: Here's the original TV ad mentioning the Beach Boys record, and they got some ad jingle house musicians to record a BB's-style jingle too lol...
https://www.youtube.com/watch?v=86F9p_rbsLA