After all, it's not compulsory to donate blood.
When I first started donating blood, my financial situation didn't allow me too much wiggle room to help people in need. I could maybe afford a box of Cornflakes every pay to put in the food bank bin.
It doesn't cost me anything to donate blood, and they give you a nice snack afterward (granted, it's so you don't faint or feel weak, but it's still a nice treat). So it was also a way for me to help other people, without causing undue hardship to my family.
I figure that unless there is some underlying health problem, most JWs who have lived the lifestyle are healthy and free of the diseases that would disqualify them from donating. You can even make your donation in honour of someone who has passed away (including JW children who have died for lack of blood) .... it's a great way of "sticking it" to the Watchtower.