MrR is an attorney in a state where he is not required to do ProBono work. However, since he has pretty much retired, that is all he does. He does it because he wants to help people, because he sees his legal work as a type of ministry not because he has to. He went into law to help people (no he is not a liberal) and that is what he does.
It is a false assumption all lawyers are only in it for the money and compelling them to do pro bono makes them want to do it less, remove the requirement and many more will do it freely.
Liberal-tarianism does not believe in pro bono requirements.