Comments of chapter undefined of Creating Manga Of One Piece In Pirates World Become My Ultimate Goal!
Ehh, the whole 'don't kill the son for the sins of the father' thing doesn't really apply in this case. Obviously Sengoku already knew Ace's parentage for a long time and did nothing about it for the 17ish years Ace was growing up under Garp. The news didn't spread either after Ace went and became a pirate before joining Whitebeard. It's only after Teach handed him over and they were going to execute him anyways that he used the news to drum up more PR and increase morale for the Marines.
I believe Roger wanted to pave the way for the 'freedom fighter', he knew he had arrived too soon and couldn't do much because of his illness. His entire family ended up sacrificing themselves for this small hope of freedom. It's not something I would do, but it doesn't stop me from admiring him for accomplishing the mission he was assigned to do.
