Fahrenheit 451

Actual copies might be discovered and taken away, whereas, memorization is in the mind and somewhat impossible to detect. People don't know what you've memorized unless you tell them. 

We're book-burners, too. We read the books and burnt them, afraid they'd be found. Micro-filming didn't pay off; we were always travelling, we didn't want to bury the film and come back later. Always the chance of discovery. Better to keep it in the old heads, where no one can see it or suspect it.

what does granger mean when he says : « we’re book-burners,too » and what made them do that?