as a former JW bible study, the reason behind that is that, the new testament wording is " to keep abstaining from things sacrificed to idols and from blood and from things strangled and from fornication" and since when a person is sick and unable to eat, the doctor usually pass food through his blood vein or nerve which equates to eating.
similarly, when the blood is pass through the vein it would be considered eating as well, as per JW explanation.
thanks