Actually, it IS exclusive.
From http://www.pennlive.com/opinion/2015/03/netanyahus_speech_wasnt_just_b.html
Article II Section 3 of the Constitution empowers the president to "...receive ambassadors and other public ministers." Furthermore, under Article II Section 1, "The executive power shall be vested in a president of the United States of America."
The power to receive ambassadors and public ministers and the executive power granted to the president means the president and only the president communicates foreign policy and conducts diplomacy between and among nations.
In keeping with the checks and balances that run through the Constitution, the Founding Fathers did include situations where power over foreign affairs is transferred in whole or in part to the Congress. For example, the president has the power to make treaties but the Senate must ratify them. The president has the power to conduct war, but Congress must first declare it.
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But the Constitution clearly makes the president the chief diplomat and sole spokesman for the United States on the world stage.
This understanding extends back to George Washington, who made it clear when a French minister, Edmond Genet, attempted to contact Congress directly to gain support for America's help in his country's war with Britain. Washington's position was that of strict neutrality.
Washington was so incensed that he had his secretary of state, Thomas Jefferson, write a letter to Genet censoring him for his conduct. Secretary Jefferson wrote to Genet that "no foreign agent can be allowed ... to interpose between him [the president] and any other branch of government." Later Washington became so annoyed by Genet's continued efforts to communicate with Congress that he had Genet recalled.
When former Israeli Prime Minister Menachem Begin came to America and blasted President Ronald Reagan's negotiations with Saudi Arabia concerning the sale of AWACS planes, Reagan was furious. Reagan shot back, "It is not the business of other nations to make American foreign policy."