YES!
He's the head of state, and regardless of the diplomatic issues of the UN, it IS on US soil and he does have to pass through US ports of entry to get to it.
The President damn well can, and the current one should.
And at the very least, he should make it clear in no uncertain terms, that Isanutjob is not welcome at the WTC.
And who cares what the UN thinks anyway.
Damn Skippy!
This is a sovereign nation