2. The second prohibition relates to flowing or liquid blood. (It is not prohibited to eat the blood which remains in the flesh of the slaughtered animal after one has done his best to remove it. (Trans.)) Ibn Abbas was asked about the spleen and he replied, "You can eat it." The questioners said, "But it is blood." (In early times the spleen was believed to be congealed blood. (Trans.)) He answered, "Only flowing blood is prohibited to you." The reason for this prohibition is both that the drinking of blood is repugnant to human decency and that it may likewise be injurious to health.
During the period of jahiliyyah, a person who felt hungry might jab a bone or sharp object into the flesh of his animal, and collect and drink the flowing blood. It was concerning this that the poet al'Ashi said: Never approach animals that are dead, Nor take a sharp bone to pierce the live one.

Thus, since piercing the flesh of a living animal injures and weakens it, Allah Ta'ala prohibited such a practice.