I don’t know how, I don’t know why, but for some reason, I have full access to Strixhaven, Descent into Avernus, and Curse of Strahd despite never having paid for them. I couldn’t find any announcements on D&D Beyond saying they were giving them away, so I’m going to assume there’s a problem with the site recognizing what I own and what I don’t own. And as much as I like the idea of free stuff, I don’t want the mods/admins to think I’m trying to steal from them, so I’m letting them know right now.

You are most likely benefiting from someone in a campaign you’re in activating Content Sharing and sharing that content with the whole campaign. As your Character is part of that campaign, you can access that content for that character and can read those compendia.

Well, I am in a Descent into Avernus campaign, so I suppose that explains it. I assume I also have access to everything else the DM purchased, explaining why I can see Strixhaven and Curse of Strahd despite not being part of either campaign.

You misunderstand the meaning of “campaign” in this context. If you are in any DDB campaign (regardless of which module it is, or even if there is no module being used) and if content sharing is enabled for that campaign, then all content owned by anyone in the campaign (DM or player) will be shared with everyone in the campaign.

You see, the system has absolutely no idea that you’re DM is using Descent into Avernus, or Strixhaven, or Curse of Strahd, or none of the above. All it knows is that you’re in a campaign with certain other users, and that content is being shared within that campaign. So it shared all content within that campaign. Make sense?