It's not about leaking your private keys (although courts very much do work like that....think of it as keeping separate keys to each room in your house, as opposed to a single master key).

The real goal is privacy, given that your public key is available on github and sent via plaintext when authenticating.

There are some added benefits to managing separate ssh keys for each server: it forces you to use tools to manage your keys, which make it easier to mitigate disaster when the time comes to rotate your keys due to compromise.