Because it's a moral relationship. It's an ethical relationship. You are given funds and you are the steward of those funds and you have to distribute them according to the best interests of the recipients. Anything else is fraud, or all these kinds of words we use to describe that.
It's an ethical relationship. It can often be a legal relationship. When first nations enter contribution agreements, they have to sign them and spend money in a certain way, depending on the grant and whatnot.
It's the same thing for politicians in the mainstream communities. It is the same type of ethical and legal responsibilities.