These folks are tax lawyers. They obey every ethical restriction placed on them by the state bars.
If they quit the federal government, where exactly are they to go to have a job?
The generalization of this principal to other government institutions (agencies, departments, offices including even the presidency) is one of the great problems of governance of our time. You can take the remuneration discrepancy as the "0=1" conclusion of a reductio ad absurdum argument against unfettered capitalism or as an argument that resistance is futile.
I do not believe there is any solution, actually, other than paying them enough to not want to go into private law firms :)
and Kevin went to Miller and Chevalier http://www.millerchevalier.com/OurPeople/KevinMDowning
Now, again, i'm a big believer in the various ethics rules bars have for what previous government lawyers are allowed to do and not do.
But they still have to be able to get jobs.
Kevin spent 16 years in government. It's not a blood oath not to go to a private law firm and be on the other side of the table, anymore than working at Google for 16 years is a blood oath to not go work at apple or facebook.
Arguments that they use their previous experience/knowledge to abuse the system (by getting favors, etc) need to be backed up with evidence, not handwaving, because they are serious allegations.
If they did, great, lock them up. But their skill sets prety much only lend them to two types of work: Prosecuting people for civil/criminal tax law, and defending people being prosecuted in civil/criminal tax law.
It's not like you can reasonably say "they used to work at the DOJ so now they have to go work at the zoo".
You don't have to go to private law firms if you don't want, of course.
But again, this argument is applicable to everything.
While working at Apple, you'll go soft on improving Siri because you may want to transition to getting paid to work more on Google Now at Google ...
Is there a situation where you think this principle isn't applicable?
(also, what is your proposed solution?)