It's apparently a case of lawmakers thinking the public can hear their promises, but can't see them breaking them. This part of the Post story sort of sums it up:
House Majority Whip Roy Blunt (R-Mo.) illustrates the complex politics buffeting the proposals to overhaul lobbyist rules. At a private meeting in Hastert's office before the speaker's news conference, Blunt voiced support, according to others in the room. But when the moment came for him to stand next to the speaker in front of the cameras, Blunt had vanished. Locked in a tight race to replace DeLay, Blunt was among the first congressional leaders to perceive the disconnect between lawmakers' public calls for change and their private desire to keep things as they were.
"This is an area where we have to listen very carefully to the members," Blunt later explained.