I find it hard to believe if a blood bank has a certain rule, they would jeopardize thier jobs over it. I know personally Houchin Blood bank will not ever take you at even 20. Red Cross has over their website that hemoglobin can't be over 20. They are the main suppliers of blood.
Your body needs iron to make new blood cells, replacing the ones lost through blood donations. Learn how to improve hemoglobin levels for blood donation.
www.redcrossblood.org
Maybe you can list these blood banks you are talking about.