Having an instance with a x million users is impossible to moderate. Instances have been blocked for not being able to moderate low thousands of users due to incompetent/unresponsive mods. Having a poorly moderated instance federate with yours is seen as a threat to your users and a proactive admin would not allow federation. Also most admins are aware of EEE tactics and we all know we cannot expect anything even remotely resembling ethical behavior from big corporate instances, they are a threat to the network by default.