Because he HAD to free them. In the context of the story this is post-Emancipation Proclamation so he legally had to free them. It makes it look like this white guy is "a savior" to slaves when it was more of an obligation.
It's like the movie "Boy in Striped Pajamas" where they made Nazis look like good guys. Would you read a "love story" between a Nazi and a Jewish person set during WW2 where a Nazi comes and free the Jewish people from imprisonment and one of the prisoner goes to him saying "I'm your prisoner"? You wouldn't because it's historically insensitive and romanticizes the bad guys.
In no way does it make him out to be a savior, he is clearly just a depressed man trying to sell his estate and move on. The slaves were never depicted as eternally grateful for their "savior" if they were then yeah I could understand that. He treated it like an obligation, and even gave them money so they can find their families and settle down.
The context is not the same, the MC hasn't had a chance to inform the cowboy of his freedom, and we never got to see him react. For all we know the cowboy left right then and there. The Nazis were actively against Jews, all of them, in some way they were the reason for their deaths. The MC did not enslave these people, he has been absent from the family affairs and clearly stated he is not a slave owner.
No I actually think there would be a bigger problem if the slaves were portrayed as white cause now the authors committing erasure of history.Ans portraying that slaves back then was white. Also just because the Mc didn't refer to himself as a savior doesn't mean he isn't being portrayed as one. Him "freeing" those slaves was some thing he was obligated to do, not something he did out of the kindness of his heart. Those slaves were already free, the only thing he had to do was tell them that.
First of all never claimed any of that, second of all like I said, if you're going to do a historical story, it has to be accurate, so you keep bringing up white people this and white people that, but you're missing the main point. Slavery is a very real thing that happened, and very real people were and still are affected by it. Not to mention you claim we wouldn't care if they were white slaves cause you can't be racist to white people, but you gotta realize that if white people we enslaved it would literally be by their own people. But the racial and dehumanizing aspects of this probably wouldn't be as heavy as it is for black people. Those whoye slaves most likely wouldn't have to fight as hard for their rights because people their kind already have their foots in the door, it would be more a classism struggle than a racial struggle. Black people on the other hand HAD to fight to get basic human right, and then for even more after that. So I promise you, this would be cared about, just not the same type of care that your ignorance is trying to block.

Okay so he let all the slaves go and even paid their expenses. He himself did not enslave them. The cowboy claimed the ranch as his own, and claimed himself to be a humble slave. So quick to jump the gun for what? Yall complain about lack of diversity, generic plots, copy and paste character designs but when you get something addressing all those. It is racist. As stated by the author, the story WAS NOT going in a master x slave direction. Which would have been clear if you actually read the chapter.