In a long article filled with details, Fred S. McChesney writes

Lobbyists are free to petition government on behalf of clients. Indeed, the process is protected by the First Amendment. Accepting money for the exercise of one’s First Amendment rights is not forbidden, either. (Abramoff’s biggest legal problems essentially concern tax evasion and defrauding his clients, not creating or extracting their rents.)

You have to read the whole article to appreciate the intricacies of the regulation of Indian casinos, and the rent-seeking incentives thereby created.