This is misunderstanding the purpose of the restriction.

The main reason not to bring a phone into the room is that the phone could be compromised. If the person is compromised then a device isn't your problem, because they could view the documents and copy them on paper or just remember the contents to write down later.

In a corporate environment no-camera/no-phone policies are sometimes also used for DLP reasons, out of expediency. Oftentimes it is more profitable to hire less trustworthy people (read: cheap labor) and simply make it inconvenient to steal data. This usually works good enough when you're trying to protect widget designs and not human lives.