For Congressional leadership, the threat of prosecution for agreeing to the The Program is enough. For the membership, the threat of retaliation by the leadership if they don't go along with immunity usually suffices. Blackmail of individual Senators by Bush-Cheney wasn't even necessary.
In intelligence lingo, to be "read-in" means that that a classified program is revealed to you, and you agree to keep it secret under the terms of a standard government security agreement.
By law, all officially acknowledged covert programs are revealed to Congressional leaders designated an intelligence oversight role. This group is known as "The Gang of Eight".
However, being read-in sometimes holds some risk. If the classified program isn't entirely legal, that potentially makes you a co-conspirator in a crime along with Administration officials. Unless, you can make the crime go away.
So it is for all involved in approving NSA Warrantless Wiretapping and CIA Torture.
It came out about six weeks ago that key Democratic leaders -- the senior Dems on the House and Senate Intel Commitees (Harmon and Rockefeller)and the Minority Leaders in the House and Senate (Pelosi and Reid) -- were read into the detainee torture program back in 2003. Along with four of their Republican colleagues. Harmon claims, when she was told about it, she opposed destruction of the torture tapes. Similarly, Rockefeller recently produced a dated letter from a locked safe about the warrantless wiretapping Program after he was informed about that in 2002 (the letter basically says he didn't really understand what he was being told and agreed to).
The fact is, the FISA revision law that contains wiretapping immunity is CYA by the leadership. It effectively grants immunity to the Gang of Eight, too. The rest of the members know what the leadership will do to them if they don't go along with passage of the immunity legislation. The House leadership has considerably less discipline over the membership than the Senate, so there may be some chance that immunity doesn't make it through the conference Bill.
We'll see. The more you know about what's really going on, the less likely they'll get away with it.