You don't understand how law firms work. What you propose would be very bad form. When you're partners with someone, you don't refuse help you're in a position to give. Now, if this group had approached him to take the case and take lead, that would be an entirely different matter. I doubt his partners would mind at all if he would have taken a pass in that case.
Please keep in mind lawyers are advocates who don't necessarily believe the same way as their clients and often take cases they might not rule in favor of if they were on the bench deciding them.
Your point on good form also applies to the attorney approaching Roberts. THe question is, did he know whether Roberts would feel comfortable in advocating judicial activism.
Your views on the divorce between attorney and client viewpoints are wuite true in the context of paying clients. It is my experience that in pro bono work, the attorney's views do tend to track with the client.