Why does the President have Diplomatic?

Answer 1

If the question is, whether the President has diplomatic immunity, the answer is no. Diplomatic immunity is governed by the Vienna Convention on Diplomatic Relations and applies to diplomats accredited to foreign countries while serving abroad (and vice versa). But the President has something equally good: head of state immunity. This protects him from prosecution in foreign countries for actions taken as President, and provides de facto but not de jure immunity from local laws. While head of state immunity has a long tradition in international law, it has been weakened in the post WWII era, when a "crime against humanity" jurisprudence has allowed prosecution against heads of state (such as former Serbian President Slobodan Milosevic). Had Hitler been captured and stood trial at Nuremberg, head of state immunity would not have insulated him from prosecution for launching wars of aggression, persecution of minorities, and mass murder.

Practically speaking, head of state immunity relates to the significance of the country in question. Great powers would not permit prosecution of their present or former heads of state in foreign tribunals. Only heads of state in small countries in which the regime has been overthrown or large countries which have been defeated in war need worry about prosecution. The erosion of head of state immunity in international law may be a contributing factor to explaining why some heads of state (like Bahar al-Sahad in Syria) cling to power--legally it would not be safe for them to do otherwise.

The President of the US remains subject to the US Constitution and US law at all times (as President Nixon discovered in aftermath of the crimes associated with the Watergate break-in). Controversy has existed when US Presidents violated the War Powers Act (Clinton, possibly Obama), but since the federal courts will not adjudicate political questions, the only effective remedy is impeachment.