You mean like the way Mary Jane let Spidey go off alone at then end for similar reasons? ;-/
Redheads you know. Once they get something in their minds... and I figure Ginny's made up her mind.
As for Snape; I think he told D about his Unbreakable Vow... D didn't want Draco to slide irreversably down into the dark side. And had to plead with Serveus to do what he vowed.
Dumbledore was a master wizard, and like a grandmaster chess player, thinks and plans many, many, many moves ahead.
The next book ought to be real interesting.
Just bumping the thread to the top