Beginning a sentence with "because"

I have excitedly begun the adventure of reading through Dan Brown's new novel, Inferno. 

Immediately I have noticed within the first 80 pages a similar layout to Dan Brown's most famous novel, "The Da Vinci Code."

Langdon meets a girl, he's in trouble, they are running away from police, trying to piece together some clues. It seems Brown is going back to the successful layout of his greatest work. And that's fine with me.

When I was at school, my English teachers always taught me never to begin a good English sentence with the word "because." So I've avoided it ever since. It was therefore surprising somewhat when I came across this line on page 28 of Inferno:

"Because the walls were nothing but metal screens, Langdon found himself watching the inside of the elevator shaft slide rhythmically past them."

Shock horror! Dan Brown wrote a sentence starting with "because!" Is he purposely writing wrong English? Or is English grammar more flexible nowadays?