Cell Phones and Driving

Go a little OT with this post.
I've very aware of big trucks.
I avoid them.
I stay back for rocks.
I don't block them in their "passing lane" if they want to go 20 MPH over the speed limit.
(They already have destroyed the far right, "truck" lane pavement).

I wonder though.
Why do truck drivers always say don't cut in front of them because they can't stop as fast as a car yet they routinely ride the people's asses who get in front of them.
And why do the same trucks that can't do 55 uphill go 90 down the back side of it.