I don't think that it matters who it was, countries are allowed to keep people out. They can refuse anyone's passport and tell them to go home, even if they have a good reason to be there. That is just how the world works and that's probably what happened to him.
The Maori leaders are justified in blocking John Key's visit. No one understands a tribe or an area more than the local leaders. Even if we don't all agree with the decision, local leaders should have the power to make decisions that they believe benefit their residents. John Key has insulted the Maori's language in the past, so it's no surprise that they don't want to welcome him.
I don't think it's reasonable for the Maori leaders to have said that John Key cannot visit Waitangi. They can't control everywhere he goes, there is some liberty that a person possess and visiting other lands is one that liberty. The vote by the Maori leaders is completely stupid and moronic.
John Key is the Prime Minister of New Zealand, he oversees the entire country and its well being is his responsibility. By signing the pact that he did, he was doing what he believed to be right. The Maori leaders also extended an invitation to him, so he had every right to be there.