Collective Bargaining for States But Not for Uncle Sam

Public employees in many states have far greater collective bargaining rights than do federal workers. As I understand it, federal government employee unions can only bargain collectively over a small range of issues. Even if Wisconsin Governor Walker is successful at pushing through his limitations on collective bargaining in Wisconsin, state and local workers in that state will still enjoy broader collective bargaining rights than most if not all federal workers.

My question is whether there is any theoretical or practical justification for treating public employee unions differently at the federal and state levels. Are there reasons to believe that collective bargaining would be a greater problem for the federal government than state governments? Could it be that states engage in some degree of jurisdictional competition and are nominally constrained by balanced budget amendments? Something else? And if there is no such justification, does this mean there should be greater collective bargaining rights at the federal level? Or does it suggest that state and local government workers should be treated more like federal workers?