Kate has a right to privacy
So do we all.
Our home is our castle. Another person cannot invade it. The powerful cannot step inside without your permission. Government must obtain a court warrant with a damn good reason if it seeks to invade your privacy. Britain's people have defended the principle of a private life for hundreds of years.
The principle applies to highly intrusive and profitable photographers with telephoto lenses.
France, Germany, Italy and the rest of Europe never really enjoying or apparently understanding this principle, the European Convention on Human Rights was established after the Second World War. (The Convention was largely written by a Brit.) The Convention declares that every person has a right to "private and family life" (Article 8).
When Kate and William are living in private, they have a right to privacy.
The French and Italians could be charged with violating their privacy under the European Convention on Human Rights.