It's not just campaign contributions though. When people step out of government they're often stepping into very cushy and lucrative jobs provided by people they helped while in government. You don't get to make that transition if your primary motive is looking out for the average citizen. It never even has to be agreed to explicitly, it's just understood by all parties how the system works. Very hard to prove corruption in that case, as it's institutional rather than individual.
Speaking personally, I'd rate that as a career choice/part of the job thing. It's like, if you've little to no interest in working for this or that large business, you've no reason to pull any punches against them - but it's a career choice at the end of the day.
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.
