Even if it's surplus to requirements? It has a shelf life, demand is variable, and it's a better fate than simply throwing it away.
I'm a regular donor in the UK, and it also happens to our donations. It can be used as is, separated into different blood products (plasma, platelets etc.), or used for research purposes. It's not realistic to expect it only to be used for transfusions; there's a lot of uses for it in addition, and I can't see why you would want to control that.
The immunology research group I used to work in routinely received blood deliveries from the NHS blood service. It was used for HIV research if I recall correctly, and was done in partnership with the NHS.
University experiments involving human specimens are not "random". They are all approved by ethical review panels, and presumably may help to save people in the future.
Because you may want your blood to be used for saving people, not to conduct random experiments.