Ok, the title is not true. But it may as well be given the mind set of some bar regulators.
What’s a Little Metadata Mining Between Colleagues? on law.com discusses lawyers’ use of metadata and that some state bars are considering making it unethical to mine metadata.
In my view, banning lawyers from viewing metadata in documents received is like banning reading a sticky note that a lawyer forgot to remove from hardcopy. The rules should work to discourage bad practices (e.g., not deleting metadata when necessary) instead of protecting those who can’t be bothered to understand the technology they use.