If you raise a lot, you get paid more. And the nonprofit may think it sounds like a good deal that if the grant doesn't come through, they don't have to pay you. While this is not binding on people who are not members, it is good advice. This may not build a good relationship for the long-term. People paid on commission have no reason to share information which could make the nonprofit more self-sufficient, but could take valuable consulting time.
Donors get very upset if they hear that the proposal writer was paid a commission.
Often volunteers with personal contacts visit the donor. In fact this is the way many of the largest donations are made from corporations and foundations. Even government grants are often influenced by who knows who. The volunteer may feel that it was her or his influence that got the donation, and resent paying a commission to a proposal writer. They pay me for the time and expertise involved in developing the grant.