The lesser fine of $5,000 might seem like more of a slap on the wrist. The $7,500 amount is more like an elbow pressed to the throat -- not enough to do any lasting damage, but enough to get a player's attention.
And that is what I think Dockett was doing on the play in question. He seemed to be sending a message, not trying to injure Hasselbeck seriously.
Additional fines from this game:
$5,000 against Grant for instigating the confrontation.
These are all coming from Darren Urban of azcardinals.com. I am waiting to hear back from the league on those and others. Will update as new information becomes available.