Jehovah's Witnesses: Why does the Watchtower allow partial blood while banning transfusions?

Their position is akin to someone claiming that he abstains from spaghetti dinner, but will eat tomato sauce, hamburger, pasta (noodles), & cheese topping all separately. Also, they oppose blood transfusions, but if no one donated blood, the “factions” they accept would not be available. It’s quite illogical.