A legal retainer publically declaring itself a member of a terrorist organization doesn't really sound like a smart idea. The government would just arrest and interrogate your legal retainer (or just eavsedrop on the legal retainer to learn that the legal retainer was smuggling messages from your Leader in prison...and then arrest the legal retainer with that charge).
It's not hard to maintain plausible deniability. The lawyer has no part in the groups crimes. There's nothing illegal about defending people in a court of law, whether they be guilty or not. And arbitrarily arresting people without any sort of due process is doubly bad for the governments image when these people can speak at length in public about why their arrest is illegal.
If the IRA can have a political party I fail to see why the LCS can't have a legal consul.