The roof is already tinted to a point, its not a sharp line, its blended in.
TBH i dont even notice it any more. Im not sure how they would get it to blend and make the transition if they tinted the rear part but they must be able to do it somehow without affecting the rest of the roof tint, or making it darker. My rear side windows are also tinted darker than my front, but i dont notice that either.
I didnt push the point, id rather have warrenty or even better no damage esp if they dont recommend it. maybe other places use different films and have different skills, dunno (I have no skills in this area, but i did watch this video on window tinting which was interesting)