Also, the definition of corruption you seem to have is not accurate IMO. Corruption is what Spiro Agnew did: trade explicit influence in exchange for some dosh in an envelope. That's not what politicians usually do today in western democracies -- which is to say, pay very close attention to what their major donors are worried about. There's a case to be made about how the two are equivalent in the end, but the fact of the matter is that in the first case the funds arrive in an envelope for the politician to pocket (minus some bagman commission) whereas in the second case the donations are (supposed to be) disclosed and (theoretically) benefit the politician's campaign.
Also, keep in mind that when a congressman leaves office, they usually have relationships with people that can move the needle in government. As such they can do valuable things for whoever is willing to hire them -- regardless of the positions they held while in office.
This seems like comparing apples to oranges. It's one thing for the EFF or ACLU to be very public about pushing this or that piece of legislation that they wrote for all practical intents and is in the public interest. It's another for XYZ lobbyists to be very private about pushing this or that piece of legislation that they wrote for all practical intents and caters to very private interests, with incidentally interested large donators reaching out to MPs about the legislation in question.
(Personally speaking, I'd love it if politicians were required, under penalty of fines and prison, to disclose lobbying efforts they were exposed to.)