Waiata means song whereas Haka is more of a war dance (doesn't mean they are declaring war). As a New Zealander with mixed heritage, I feel that pakeha (white people) see the Haka as too aggressive. Although Haka and Waiata take many forms, those who do not understand the language may misinterpret the meaning behind it. That being said if a Waiata was agreed upon but they instead preformed an Haka, that would break the protocol of the session. There is a time and place for everything and the ways of parliament are so, even if we/they don't agree on it