In Georgia, the police are accorded special rights during grand jury investigations — rights that are not available to ordinary citizens. First, an officer can attend grand jury proceedings. Second, an officer can bring his lawyer into the grand jury room. Third, the officer's lawyer can cross-examine the state's witnesses. Fourth, an officer can make a "statement" to the grand jurors after the prosecutor has finished presenting his/her case. (See Title 45-11-4 of the Georgia Code).
A case can be made that those special procedures can help a bad cop avoid an indictment or conviction. On the other hand, a case can be made that prosecutors have too much influence over grand jurors and that those procedures simply make the process more fair and balanced. Whatever the merits of those arguments, the double standard is inexcusable. If anything, the police should be held to a higher standard than John Q. Citizen.
I can't conceive of a remotely convincing justification for the double standard.