When big law firms do pro bono work they assign the task to a first or second year lawyer. But they provide the assignee with help from their senior lawyers.
It is in the best interest of the law firm to provide as much assistance as they can because it is providing training to their newer lawyers, i.e. making them better lawyers.
Since Roberts was a member of the firm, it was in his best interest as well - in strengthening the firm. I suspect that that is just what Roberts provided.
It has been said that he participated in a moot court. Based on his comprehensive experience with the Court, he was very well qualified to do that. That's how law firms work. It certainly wasn't Roberts' fault that the opposing lawyer was not as well prepared.
That's how I read it.
The other point is that law firms are partnerships and clients are clients of the FIRM, not just one of the lawyers in the firm. If a lawyer had to conduct an ideological litmus test before deciding if he or she was willing to provide help within the firm for someone else's case, he or she wouldn't last very long.