I don't understand the question. What's a moral mechanism? If you are asking for a general moral principle that establishes the obligation, I should point out that I'm a moral particularist. I don't think that appeal to general principles is essential for sound moral reasoning, and I don't think there are many true, simple and general moral principles.
But surely you recognize that the cases share internal similarities even if you want to distinguish them?
P/S/A: There are single sentences which can create life-changing amounts of difference.