There's always a limited number of free tests available but how many are wasted* on people with the wrong blood type? The tests are expensive and if someone knows up front they're not compatible but can afford to pay for a test, they might instead donate funds instead to purchase free tests for those that might be compatible but otherwise couldn't afford to get tested.
Here is who can donate to whom with regards to marrow transplants:
Type A can donate to types A and AB.
Type B can donate to types B and AB.
Type AB can donate to type AB.
Type O can donate to types A, B, AB, and O.
*I know that tests aren't wasted because the people are entered into a DB that benefits all marrow transplant recipients but the individual appealing to others for help could improve their odds.