Things you can't say

The NZ ambassador to Canada has caused quite a flap of left-wings by saying that Maori tribes in New Zealand fought and ate each other. The exact words he used were:

"There were seven canoes that came in 740 from Hawaiki, so there are seven tribes. They all held each other's hands to stop them from sinking on the voyage. Once they got to New Zealand, they started fighting and eating each other so there have been Maori wars ever since then"

While this may be overly simplistic, it is essentially true. But of course you are not allowed to say this. However, it is permissible, even encouraged to wallow in shame about all the bad things that colonial Europeans did when they arrived in NZ.

Lets be realistic. Both Maori and Europeans did some pretty awful things, both to their own people and to others, but this in no way reflects on people today. Why is it ok to dwell on historical offences by one section of the population but not another?
Can't we all just get along?