Because it’s what we’ve come to expect from large corporations suddenly joining the table of any FOSS project that is adjacent to their financial stakes. Coexistence is possible if they can profit from the software without assimilating it, but it also stands to reason that they will be pushing for new interoperability standards that benefit their own business model at the expense of users in some way.

The lowest hanging fruit would be something that allows them to associate Fediverse accounts with users whose marketing data already exists in their database, or providing a service to third parties that helps them tie their own databases back to Fediverse users. This would require some sort of hook that encourages the users to either associate their Fediverse accounts to an existing Meta service, or otherwise volunteer common PII such as email address that can be cross referenced. Maybe some kind of tracking cookie that accomplishes the same.

Keep in mind that this is just an example, it is not necessarily the exact angle they are pursuing. I’m not in the automatically defederate camp, but a healthy amount of skepticism is definitely warranted.