There are good reasons not to federate, Legality is kinda murkey when it comes to stuff like this. Federation could (not saying it would) be used as a weapon against them. I mean, there are all sorts of instances on the federated networks, many of which do not comply with US law. Not just the real shitty stuff, But even instances that just don't care about DMCA for instance. It could simply be that Federation is just way too much of a hassle, which I really couldn't blame them. I did try to host my own federated service instance and it was just kinda too much.