What do you mean "nobody ask the reasons.."?
It was a healthy idea to get these kids away from the reservation ghetto. Give them education, incorporate into the society.
The wording was questionable by today's standard - "To kill an Indian in these kids". But the idea was give them an equal opportunity.

Well, the implementation was flawed. As implementations always are.

But this all is common knowledge. There's nothing "to question" here.