The American Thanksgiving is mostly descended from harvest festivals, which are hardly a rarity in cultures across the world. The earliest Thanksgivings held by colonists in what is now the United States have certainly been embellished, and the historical context in which they took place has often been whitewashed, but that doesn't necessarily making the celebrations themselves a bad thing.
The colonists were not celebrating genocide, intentional or accidental. Indeed, if any aspect of their relationship with the Native Americans was being celebrated it was friendship. Yes, that friendship didn't last long, and colonists from Europe in general did horrible things to the Native Americans both before and after the first colonial Thanksgivings. But ill will toward the Native Americans was neither encouraged nor celebrated by any Thanksgiving accounts that I could find.
No, Thanksgiving was about celebrating the colonists' harvest, their survival, their community, and their religion. The religious part isn't very prevalent in today's celebrations. Either way, Thanksgiving doesn't celebrate atrocities against Native Americans any more than, say, Christmas does.
The problem, I think, comes when the Thanksgiving narrative is used as the _only_ reference to native-colonial relations. When a lot of horrible things that happened are forgotten or ignored. Thanksgiving, if anything, celebrates positive relationships between natives and colonialists, but exclusively focusing on this is problematic because such relationships were in the minority.
If the full story was more widely known and addressed, I don't think that Thanksgiving would be a problem at all. It's actually a holiday that I enjoy quite a bit, and most of what it actually celebrates is worth celebrating in my opinion. It just shouldn't be used as an excuse to ignore all the other horrible things that did happen. When it is used in that way, I agree that it is insulting and awareness should be raised. But Thanksgiving can, and should, coexist with awareness.