Andrew Gelman rises, somewhat, to the defense of Evan Bayh:

[T]hink of this from Bayh's point of view. After being one of 100 U.S. senators (and near the median, at that), it's natural to want to stay near the action and have some effect on policy. Lobbying is a natural way to do this. From this perspective, it's a direct extension of what he's been doing before. And if it pays well, so be it.

As a Senator, Bayh's role was to influence policy on behalf of the citizens of Indiana and Americans as a whole, and if those policies conflicted with certain narrow interests, so be it. As a lobbyist, his job is to influence policy on behalf of whoever pays him, and if those policies conflict with the interests of Americans as a whole, so be it. Both those jobs constitute "having some effect on policy," but the goals are, in theory, quite different. If Bayh's behavior as a private lobbyist is a 'direct extension" of his public service, then that is a devastating indictment of him.