The metaphor implies they were feeble minded. That's a pejorative insult.

" what actually happened to the natives "

According to the propaganda you've been sold.

When you think of "scalping" the first thing most Americans think of is those "savage Indians" because that's what they were taught - meanwhile it was the US Government who offered bounties on the scalps of Native Americans. Of course the propaganda flips it on it's head to make the white people feel good about it while they continued their broad systemic horrific abuses to the "bad guys" well into the 1960s - and even up to today.

https://DuckDuckGo.com/?q=US+Government+bounty+on+scalps

Considering that they were systemically hunted and slaughtered, would you say the bargaining table was fair or equal? On most of the "treaties" there was the threat of "accept or else" - and that hardly counts and being simple minded and tricked.

Eventually the government doesn't even ask anymore. They ignore the Dakota Access Pipeline protests and roll in anyway. Fuck Obama. Fuck Trump.

https://duckduckgo.com/?q=NoDAPL

A perfect parallel is what's happening now in Palestine. They are not simple minded nor being "tricked" out of their land. They are systematically being murdered and the land is stolen from them on perpetually broken "promises". Fuck Israel. Fuck Zionism.