I have access to the Spelljammer and Dragonlance sourcebooks in DndBeyond without having purchased them.

I have purchased physical copies of the books, but not the digital books. 
As far as I know, DNDBeyond is not aware of my purchases from my local game store. 

At https://www.dndbeyond.com/account/marketplace, I can see my purchase history. I have not purchased these sourcebooks according to my purchase history. I have purchased many other sourcebooks and packages. 

As far as I can tell, the entire sourcebooks are not part of a monthly subscription bonus or package I've purchased. I've received no gifts to my DndBeyond account. 

Why do I have access to these books? I'd happily just use them, but I'd hate to have the access eventually revoked and miss out on the current advent deals). 

You must be in a campaign with someone who has purchased them and content sharing must be enabled for that campaign.

Quote from IamSposta >>

You must be in a campaign with someone who has purchased them and content sharing must be enabled for that campaign.

 This.

I expect if you try making a character that isn't in a campaign, it wont have access to any races or backgrounds from these books.

That's exactly it - thanks all!