On the other hand, if the mail carrier looked at the content of the message and through some algorithm decided that everyone on your street might want to see it (or more accurately, that if everyone on your street saw it, it might boost their advertising revenue) and put it in every mailbox they could, I think they would shoulder some blame.

I was thinking of 'distributor' in the sense of supplier. Even so, I think a postal service has some obligation. Let's say someone were to send me a poorly camouflaged mail-bomb. I wouldn't expect my postal worker to just toss it on my front porch with an 'eh, not my problem!'