Buddhists contemplate death on a daily basis, to a degree that makes most non-Buddhists very uncomfortable. While we'd naturally be attracted to donate organs to benefit a few others, what can be accomplished at the time of death if the body is undisturbed will benefit countless beings.

I think there are valid reasons for people who don't wish to donate their organs, regardless of religion, and I don't think it's beneficial for us to judge them for it.