@plasma4045@mas.to probably legalities, because this is a social media site just like Facebook. I don't think it would look good on them harvesting data from other social media sites "without permission".
They'll just say that you passively permit them to do it since you stayed on a federated server after it was known that they'd do it.
Also, probably because the admins that are under NDA, aren't under NDA for there health. Health of their pockets, more like it.