My opinion: They are not being unreasonable. In fact, they could be far more restrictive and claim your photography as moonlighting*. If you are taking shots on their time and dime, they have every reasonable expectation of ownership. Not unlike the designs you do for them as an architect.
Whatever you do on your own time and dime, is your business.
*- Some companies will do this, and I personally disagree with it (as long as their is no direct liability to the employer), but it is not illegal to have such a policy either.