He's the Head of Government of a UN member state, even if they were minded to it would be diplomatically unacceptable to deny him access. The USA even let the bloke below fly into JFK and give a speech, despite the fact that the real JFK had tried to murder him
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As to the issue of criminality, there are plenty of other leaders who preside over torture and murder, you'd be left with San Marino and maybe Cape Verde if they banned them all.