And if you're on the cellphone doing whatever, you should be shot.
FTFY.
And by the way, the laws in some (probably many) states do state that you HAVE to go the speed limit, with a few exceptions. For example, here is the text from Arizona law:
"A person shall not drive a motor vehicle at a speed that is less than the speed that is reasonable and prudent under existing conditions unless the speed that is reasonable and prudent exceeds the maximum safe operating speed of the lawfully operated implement of husbandry."
"The speed that is reasonable and prudent under existing conditions" is defined elsewhere in the law as the speed limit if there's no bad weather, road hazards, etc.
Most people driving below the limit would argue that it exceeds the max safe operating speed of the vehicle, but in reality most newer vehicles can drive the limit just fine. It's the driver who's not comfortable with driving the limit, blaming the car is just a convenient excuse.
Of course, this provision is never, ever enforced. But it should be pretty obvious why it's there. People driving 20 MPH under the speed limit add nearly as much danger to the roads as people driving 20 MPH over the speed limit.