The short version:

I was leaving a shopping centre parking lot and I was pulled over for not stopping before driving onto the public road but was not charged.

The long version:

I and two other cars behind me drove straight onto the roadway making a right-hand turn from a parking lot. The way the parking lot entrance and road are oriented made for perfect visibility (you could see way down the road without even turning your head). After we all made the turn, a police car coming the other way turned on it's lights and I pulled over. To my surprise, the officer rolled down her window and started yelling at me. "What do you think you're doing?" she yells. "You have to stop before pulling onto the roadway." Then indicates she's going to get the other two vehicles behind me but doesn't. She then pulls in behind my car and and comes to get my info. She seems really angry so I just co-operate and give her my license, reg, and insurance. While she's checking my info, other cars are entering the roadway directly in front of us without stopping. Soon, another police car pulls up beside her and they talk for a bit before it leaves. She comes back to my car and gives my license back and says she's not going to charge me because she has another call and that I should consider myself lucky. She seemed really agitated still so I left it at that.

So the question is, what law did I break? I looked through the Highway Traffic Act and the only thing I could find was section 139. "Right of way on entering highway from private road" however there weren't any other vehicles or pedestrians in the vicinity. There were numerous other vehicles doing exactly the same thing I did right in front of us and she didn't bother with them. Did I do something wrong? Maybe she was just having a bad day and made a mistake?

Unless their was a stop sign I believe you are only required to yield when exiting a private drive, I was always taught that if the way is clear you can go without stopping.

Was there a stop sign or traffic light?

There was a stop sign on private property. Not a public road sign.

The stop sign may still be on city/town property. Most municipalities have a large allowance of property well back from most roads to allow for infrastructure under the ground. You'd have to do some research to find out in each case.