Most of us aren't pure-logic Kantian robots. We have a goal in mind that's beyond simply being logically consistent. When we form an organization that sometimes defends the Nazis, we're not doing it because our only motivation is that some rights are absolutely inviolable no matter what. We do it for mostly pragmatic reasons, such as the one you've outlined. If the Nazis get a fair trial, then that helps ensure that we also do. If we make too bad of a law to use against the Nazis, we could end up as victims of it some day. Defending the Nazis is ultimately a means to the end of achieving a good society for us to live in.

That's why whenever the true end of doing such things starts being endangered, principles start faltering.