From a statement published by the State Department's protocol office:
As a result of the legal reforms enacted in 1978, members of foreign diplomatic missions are accorded only those immunities which by general agreement are required in today's world to protect the interests of their governments. The number of individuals afforded full diplomatic immunity is considerably limited under the Vienna Convention of 1961, which makes the degree of immunity commensurate with the rank of the member of the diplomatic mission.
It is well to emphasize that diplomatic immunity does not exempt a diplomatic officer from the obligation of observing local laws and regulations. An envoy is not clothed with diplomatic immunity to enable him to indulge with impunity in personal controversies or to escape from liabilities to which he otherwise might be subjected. The purpose of immunity is not to benefit individuals but to ensure the efficient performance of the functions of the diplomatic missions.
When abuses of immunity occur, however, the United States is obliged under treaty and statutory law to recognize the immunity of the offender, without in any way condoning the offense. The Office of Protocol in the Department of State is available to both citizens and diplomats for the resolution of problems which may involve diplomatic immunity. That office has quite often been successful in settling disputes brought to its attention where the immunity of an individual has prevented a satisfactory settlement through the judicial process. . . .
The immunities and safeguards provided in the Vienna Convention are precious to the United States. They shield our own diplomats serving abroad from prosecution by local authorities operating under judicial systems with minimal due process safeguards. Without such protection it would be difficult, if not impossible, to staff our posts abroad.
It is necessary, therefore, to balance the interests of the United States by extending privileges and immunities to foreign diplomatic personnel here so as to ensure that our personnel are accorded the same treatment abroad.