It's difficult to discern a legitimate purpose or necessity for this. The Justice Department has all the power it needs to investigate alleged crimes by attorneys for detainees -- a power it does not seem to be shy about exercising. As I noted here, Special Prosecutor Patrick Fitzgerald is investigating the alleged involvement of attorneys with the ACLU's John Adams Project in outing covert agents. (If only the administration were equally enthusiastic about accountability for agents who engaged in torture or government lawyers who justified it.) With a few exceptions, the Fitzgerald investigation has, so far, generated relatively little publicity. (Newsweek reported on it here, the Washington Times offers a right-wing perspective on it, firedoglake.com debunks it from the left.)
Who knows how the investigation will progress or conclude? But it could conceivably prove less consequential, and less damaging to civil liberty, than proposed legislation empowering the Defense Department to investigate irritating defense attorneys who interfere with its work (as committed advocates for detainees are likely to do). This bill demands extensive scrutiny.