Ok.
If there is a blocker in front of a runner, and you go down in front of the blocker...you are going to get flagged. That would happen EVEN IF you don't contact the blocker...as you block the blocker from his advantage...and you can't do that.
If there is no blocker...you can go low for the tackle, as a quarterback, and save yourself the shoulder impacts.
Are you sure that's the rule? I don't think Hasselbeck even touched the blocker. He made a clean tackle.