The exact details of 1621 are debatable, but I would just offer:
1) The spirit of Thanksgiving is generally good, even if there are historical doubts, so we can maybe understand those that want to embrace it. It's a heck of a lot more positive than most other holidays. People actually show love, feed the homeless, etc.
2) Native Americans absolutely positively are entitled to see it negatively. Their people were nearly eradicated through horrific genocide by settlers, whether or not some Pilgrims celebrated with them over turkey. Thanksgiving wasn't an actual holiday for another 240 years, during which time they were overwhelmingly treated as "savages" and murdered for their native land. There is no defense for insisting someone with native blood embrace the day personally.