How does a Trezor passphrase prevent restoration of a wallet if an attacker knows the seed?
The seed is protected with a password or PIN; it precludes the possibility of stolen devices being decrypted whilst online malware/rootkits are precluded by the hardware device being separate from the compromised OS's memory pool.

From Trezor:


  TREZOR’s PIN based security system is a powerful tool to keep your
  bitcoin’s safe. If your TREZOR is stolen, the only thing between the
  thief and your bitcoins is your PIN and/or passphrase. It is
  imperative that you choose a good PIN!