Actually, I really think they didn't know. I've seen them; talked to some. They thought they knew. They knew it would limit ads critical of them. They were told - by the bill's sponsors, McCain Feingold et al. -- that the bill would keep groups like the Club for Growth and the NRA from spending big bucks in "their" campaigns. They were told that it would penalize millionaires who would run against them. But they really weren't told - and didn't read - about how it would limit their own activity.
Our government is too damn big, and members of Congress can't begin to understand, let alone read, all that they vote on. What happens is that on any issue, a small number who are particularly interested really become experts, and the others defer to them. So you're a congressman, and a bill comes up on, say, regulating phone companies. You go to the congressman who thinks like you on most things who has learned this bill, and he tells you how to vote. Then a bill comes up on say, bankruptcy, and you're the bankruptcy expert. The other guy comes to you.
Campaign finance is really boring. Their eyes glaze over. They just trusted McCain and Feingold and Shays and Meehan. But you see, here the system broke down - because Feingold is a true zealot, and the other three - especially McCain - really didn't understand their own bill. My guess is that McCain, Shays, and Meehan have never read the bill. They sure as heck didn't write it - it was written by lobbyists for Common Cause. So members went to these guys, and these guys themselves didn't really understand it. And now they've screwed themselves and us.