I kind of agree it is a special circumstance. Instead of being wantonly reckless and breaking the law by taking a bribe while in office the work around is an incredibly high paying job with perks when your stint in Congress is over. I'm sure over time without implicitly making a deal there is an understanding about supporting/promoting/introducing or fighting legislation before the congress and what a congressperson will be getting for it in the future. Just as someone who volunteers to join the military gives up certain freedoms/rights to do so I don't think it's not unreasonable for someone seeking office and the power that comes with it to agree with a unique set of restrictions as part of the package.