The Tower of Babel represents confusion. Beatty's society in Fahrenheit 451 doesn't want people to think. It doesn't want confusion, which would make people have to think and make decisions.
Books that offer different opinions would lead, according to Beatty's view, and his society's view, to people having to think and decide.
Beatty's society wants simple-minded people who will do what they're told. Confusion would destroy this.
Beatty and his world will not tolerate differences of opinion, and thus, will not tolerate books.
The reason that Beatty says this to the woman is because, in his opinion, the books do not agree with each other. This means that they would not really be able to understand one another, and that is why they are like the Tower of Babel.
In the Bible (the book of Genesis, to be exact), a group of people were going to build a tower so high it would reach Heaven. God did not like them trying to do this and so he made it so that they all started speaking different languages. When this happened, they could not longer understand or talk to each other.
So it is with the books. They don't agree, so they can't really talk to each other.
When Montag and the other firemen report to a woman's house that is filled with books, they find that the woman has not left her home as ordered. She insists that the men can never have her books, and she will not leave them. Captain Beatty reminds her of the law.
"Where's your common sense? None of those books agree with each other. You've been locked up here for years with a regular damned Tower of Babel. Snap out of it! The people in those books never lived." (p. 35)
Beatty draws a parallel between the various ideas contained in the literature of the past with the confusion that occurred at the Tower of Babel as people began to speak in different tongues and no one could understand any one else. Books, he contends, contain different perspectives and thoughts on the issues of life. There are authors with conflicting theories and thoughts, and these differences cause discontent, Beatty explains, because no one who reads their works knows what to believe.
"And if they're nonfiction, it's worse, one professor calling another an idiot, one philosopher screaming down another's gullet. . . . You come away lost."(p.59)
Clearly, the society of Montag and Beatty is one that desires unity of thought because differing ideas often effect change, even revolutions. Independence of thought, which is fostered by such books as Beatty mentions, reduces the power of a government's control over the minds of its citizens.