Why Queen Elizabeth II vetoed Prince William's pre-engagement plea for Princess Kate

Prince William made a request to protect his girlfriend, Kate Middleton, now the Princess of Wales', security before they were engaged, but it was vetoed by his grandmother, the late Queen Elizabeth II.

Why Queen Elizabeth II vetoed Prince William's pre-engagement plea for Princess Kate
Prince William made a request to protect his girlfriend, Kate Middleton, now the Princess of Wales', security before they were engaged, but it was vetoed by his grandmother, the late Queen Elizabeth II.