No. A defensive player cannot cut down a blocker by hitting them low. A defensive player can hit the guy with the ball pretty much anywhere. Just no helmet-to-helmet contact.
The call came because Hasselbeck cut low in front of a Steeler blocker in order to tackle the guy with the ball. Bogus call no matter who you're pulling for.
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.