There are specific rules about open field tackling in a situation like that. You cannot tackle below the knee on returns.
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.