Not a lot is harvested past Samhain, unless something really wrong is happening. Having worked in the organic agriculture industry, I know that any harvest festival should be in October, like the Canadian Thanksgiving.
Also, historically speaking the early settlers really screwed over the natives. Some call Thankgiving a day of mourning because of the historical facts.
However, in modern times, I think that the idea of whites cooperating with natives is a good sentiment and should be lauded. So, sure, historically, things ended up bad, but for one day in the year, we can look to the ideal of cooperating between people of different cultures. Even if it is a fiction.
Also, it is a time for families to come together.