Actually I think the moment you take a camera out.... you will very soon find someone (normally security guard) walking towards you.
I have been detained for taking photos of colonial houses by the police (The houses happened to be rented out to US servicemen which I didnt know about)
I have had Prada sales staff come out of the shop at Paragon and stand in front of me blocking my camera telling me no photos just because I was 10 m away on the public pavement holding a compact camera pointed in their direction. (I was taking a photo for a foreign friend who wanted to show that she has been to Orchard)
I have even gone to make a police report to report a found wallet and have the police search my bag and then say that they would have to detain my camera and return it to me when I leave the building. (For security reasons which I understand)
In short.... its something that we have to get used to I guess. These people do have every right to stop us as they have a job to do. And its not only about whether you are a terrorist. One amicable way of settling it could be to discuss nicely with the management and then arranging for a day when you have made an appointment and the security have been informed of you visit to take the photos in peace. No point arguing with the guard about copyrights and such. Firstly he or she doesnt know what you are talking about, secondly he or she doesn't care, you are standing in their property and their orders are to protect it both physically and visually. Both parties move back one step, Im sure things can be worked out.