I had a similar experience also, but at CHIJMES. And I wasn't event taking a photo, just sitting down at somewhere secluded doing a ink and watercolour sketch of the building. Security stopped me.
Reason: No commercial activities allow on private property, photography or whatever.
I just left as I don't want to waste time arguing with people who don't have common sense.
Anyway, the real reason is those property owner don't want you to make money without giving them some.In Singapore, our exams do not test for common sense. When in doubt, follow the rules, follow the hierarchy, ask for permission, check back, follow orders, do not use your initiative, do not listen to reason, do not care whether you look stupid or not.