Pennsylvania banned most imports of firewood from out of state yesterday. Here is a report from TV station WGAL. The goal is to prevent the spread of parasites, such as the emerald ash borer that hitch a ride in the firewood. They could have banned just ash wood, but because it's hard for most people to identify species, they banned all of them.
In June, the PA Department of Agriculture announced the discovery of infected ash trees in several counties of western PA.
The emerald ash borer was first discovered in Michigan in 2002 and is heading this way. It has the potential to destroy most of the ash trees in American forests.
Fortunately, firewood tends to be marketed only locally. Firewood is bulky, heavy, and has relatively low value compared to other forest products. It doesn't really pay to transport it very far. But I can imagine a scenario such as campers bringing a small quantity of wood for a camping trip. It only takes a few infected pieces to spread the insects.