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.