Even if you hold that opening the fridge is assur, lichora there is a loophole. If you ask someone else to open the fridge and he doesn’t know that there is a light that will go on, he is misasek.
This happened to me a few weeks ago, and I had that same theory, and I asked my rav. He disagreed. He said it was not misasek. Misasek, he said, is when you don’t know you are doing something. Unlike here where my sister in law would have known she was opening the fridge.
I didn’t really understand what he was saying though. Seemed to me that my sister in law would not know she was turning on the light.