Mrbunyrabit said,
"That means that witnesses are in fact profiting from other peoples sins.. Because those other people are going to lose ever lasting life because they donate blood.. but witnesses can use that blood?"
You make a good point here. JWs are allowed to accept blood fractions (it's a conscience matter), and all blood fractions are derived from someone else's whole blood donations. Witnesses can't give blood, but they can accept fractions from other people's blood.
"How does that work?"
It works for no other reason than the Watchtower Society says it works. It is not Bible based. It's one of many contradictions in the Watchtower belief system.
Welcome to the forum. You have noticed an issue that seems to go right over the head of most JWs.