Some weeks ago my sister asked this question, and the elders answered it based on an Old Testament principle. So, according to them, Jehovah let israelites to sell carcasses of unbled animals to foreign gentiles. So, the same principle applies for the current people of God. Witnesses can take the permitted blood fractions from wordly people, but they (the witnesses) can't break God's law on blood by donating it.
That makes no sense at all.
Old Testament principle: Worshippers of Jehovah can give unbled meat to gentiles
Modern day application: Worshippers of Jehovah can receive blood fractions from unbelivers
In what possible sense can "giving away unbled meat" even be remotely associated with "receiving blood fractions"?
It is so nonsensical that even WTS writers must recognize its stupidity, since this idea has never appeared in print. If those of such dubious writing ability recognize it's a stupid application of "principle", that's a pretty good indication that it really is a stupid application.