Just one factor: wind direction and intensity changes constantly and isn't normally uniform to begin with on that scale.Good point.
But the forest arsonists could just move over a state or two, and repeat the linear attack every few days until much of the west was burning.
Especially upwind of cities and towns. I think another "Denver fire" could be ignited every day, one arsonist could set ten fires on a line 50 miles long upwind of a city in two hours.
What's to stop terrorists from doing it, other than "hoping for the best"?
If arsonists went wild in that manner, no doubt the state police and other police agencies including feds would be all over the highways and in the air taking a good look at any vehicles that were out of place, like stopped where people don't normally stop. The arsonists might set a few fires, but they would all be picked up in short order. I hope.