You're correct, ReelJustice.
Universal can absolutely tell you that you can't take photos of a certain area. It's private property -- they can do what they wish. They can also kick you out for wearing orange if it's a "No Orange Day". They can pretty much have whatever rule they wish to have as long as it's not discriminatory.
I know a lot of us have an entitled "It's a free country, I can do what I want, or else let me speak to a Manager" attitude, but unfortunately, it's just not the case.
Listen, in construction of new theme park attractions, there are new technologies and trade secrets being put into action. You, taking pictures and posting them online, can alert the competition to what Universal is doing. I'm sure you wouldn't like it if park XYZ also opened a similar attraction nearby. It makes it not as special. This is the theme park equivalent of you going to Apple headquarters and taking a picture of the next iPhone.
Sure, there's also security risks with third party vendors and whatnot. But ultimately, Universal (and any smart company) must protect their millions of dollars of investment, and try to minimize any breaches of their intellectual property -- including you taking pictures of the construction activity.
And the "Any press is good press" philosophy does not apply here. The pictures posted on this site are not drumming up publicity and selling tickets -- everyone here is a theme park fan and is already going to the parks.
I'm not shunning the picture-takers and information-sharers on this board. I love it. But just don't have an aire of entitlement when someone asks you to stop taking pictures in their backyard.