I would make this assumption in this case.

You think people who would opt-in register for organ donation would be less likely to die in a way compatible with kidney harvesting than others?

I think it's somewhat unlikely that whether or not someone registers to be an organ donor would affect how they are likely to die, but if it did, I would wager that registered organ donors are more likely to die in a way that enables their kidneys to be harvested than others. In any case, I doubt the difference is more than, say, a factor of 2.