How is this pro bono? His firm is being paid.
My guess is that Roberts the was only one at the firm with experience of arguing cases before the highest court in the land.
When a firm works pro-bono, they do not get paid by the client. However, if they win the case, the losing party often myst pay the legal fees of the case; this is especially true when suing the govt.. The firm was paid by the state govt. for expenses (including lawyer fees).