How do non-deterministic wallets violate the principle of address reuse? In chapter 5 of Mastering Bitcoin, Andreas writes:

"The disadvantage of random keys is that if you generate many of them you must keep copies of all of them, meaning that the wallet must be backed up frequently. 
Each key must be backed up, or the funds it controls are irrevocably lost if the wallet becomes inaccessible. 
This conflicts directly with the principle of avoiding address reuse, by using each bitcoin address for only one transaction."

If we are using each bitcoin address for only one transaction, how does this result in address reuse?
The text indeed doesn't make it's case clear, but the idea is that random key generation makes for inconvenient backups.  The more keys you generate, the more frequently you will have to back up the wallet.

To avoid this, some users of non deterministic wallets have resorted to reusing keys\addresses, as a sort of easy way out of having to follow the demanding practice of frequent wallet backups that are needed when new keys are generated.