Because they can absorb the loss of Miller. Losing horvat and keeping miller will cripple the franchise imo. On top of being an elite faceoff guy, good two way cman, the leader on the team, a consistent offensive producer, he's now added top level goal scorer to his resume.
And if you think Millers attitude stinks now, wait until horvats gone and miller realizes he's signed on to a team going in the opposite direction he wants his s career to go. He'll be intolerable on the ice and in the room. How do you think Petey is going to react to that?