Adversarial justice systems require that people have a right to be defended by a lawyer when tried by the state in a court of law.

This is because court procedures are complicated and idiosyncratic and most people would not be expected to have the skills to defend themselves. The state is trying to take away a person's freedom so part of the social contract is that it has an obligation to provide them with independent help to navigate the process.

As far as I'm aware, Minsky had no legal case to answer and Stallman was not his defence lawyer. So while Stallman certainly has the right to defend him, in doing so he was risking his own reputation in a way that a criminal defence advocate (even when their client is found guilty of the most heinous crime imaginable) does not.