Here's the thing: you're sitting on that lady's property. If she wants you gone, she has every right to tell you to leave. It's the same thing as when someone wants to sit in your yard: if you have a problem with that (which would be very understandable), you have every right to tell them to take a hike.
Of course, I don't think it's very nice that this lady made you leave, and it's certainly not good business practice. But legally, they can enforce any rules they want on their own property.
Once a business tells you to leave, remaining would be trespassing. So the initial sitting would not be illegal, but once you were told to move, failing to do so would become illegal.