Yes that's true. But as is the case in most of the universe, rules only keep the honest people honest. If I wanted to I could do a reset and have the camera active on my phone. If I get caught then I would lose my job. But unless security is willing to do a full cavity search on everyone, everyday, then there is always a security risk. Blocking the camera phones reduces the risk to a reasonable level.
And more importantly, satisfies the customer.