If you're dead, what's to own? If anyone takes specific exemption to it they could opt out - but for everyone else we address a legitimate public concern (lack of organs for donation) by accessing an otherwise wasted resource.

And it's not like this is a readily producible resource - people die on organ transfer lists precisely because other avenues of acquisition can't meet the necessary levels. It's a question of priority IMO, maybe not philosophically ideal but in a cooperative society compromise can save many lives.