> Why should the fake accounts team care about the metrics of the ads team?

Ok, now I understand what you meant. In this case it's because the strategy comes from high up and (putting plausible deniability of individuals like FB's CEO aside) the manager of the "fake accounts" team got some instructions from above to focus "here" not "there", and this just happens to be in the best interest of the "ads" team.

The employees themselves may be individuals but they're there working for the vision of the company and CEO (because it's usually the CEO who sets the course and is aware of all these directives). They're not doing it for the other team, they're doing it because whoever set the strategy decided it's in the best interest of the company.