The paparazzi isn't entitled to harass celebrities under the First Amendment. The first amendment only gives us the right that we can't be prosecuted by the government for things that we say. The paparazzi would be only prosecuted under the law if they harass celebrities, which is a different story entirely.
Yes, the Britney Law is consistent with the First Amendment, because it balances another equally important right, which is the right to privacy. A famous person has a right to travel without harassment, and it is important that they can do so in society without fear or interruption. Although others have their rights as well, limits can fairly be placed on the First Amendment in order to protect other rights.