Snape really didn't want for Sirius to apologize either -- it was already awkward that he nearly had and frankly neither of them were really in a place where it would mean anything anyway.
Why Black had done what he had and how was obvious -- that wasn't really in question at this point -- only the man they'd trusted that Secret with was.
"That isn't how it worked out," Severus said with a small note of finality. There wasn't much point in talking about what they would have done, since the deed was done already and nothing would take away the ache of what no longer was. "But I'm aware. It should have been obvious."