The narrowness of career paths for highly specialized attorneys combined with the possibility of entering an altogether different remuneration universe at the law firms the DOJ is theoretically adversarial to is what makes the revolving door spin, and the spinning of the door is what turns "adversarial" into "theoretically adversarial". That is the problem. I didn't say there was an easy solution.
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.
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?