Roberts was a partner and one of the top appellate lawyers in the country. He had a choice.
That said, more needs to be learned of his actual work and role on the case.
Hogan and Hartson has over 500 partners. Hogan and Hartson is so big that they have a pro bono group within the law firm.
It is the law firm of Hogan and Hartson and a consensus of the 500+ partners in that firm who decide what Pro Bono work is done by the firm.
The then-head of the pro bono group of Hogan and Hartson asked Roberts with all of his appellate experience to review the filings. Roberts obliged. There are many possible reasons why Roberts did so including the reason of loyalty to the law firm that he worked for.
The title of this article is misleading. All to many on this thread are all to eager to be mislead.