As far as the "legalities" go, my thought would be that the school owns them. The PTO is an entity of the school, whether or not it's an independent 501(c)3. Without the school, there would be no PTO. The fundraising dollars that the PTO earns are then given to the school in one way or another, be it purchasing appliances for the teacher's lounge, or paying an assembly speaker, etc.
I wouldn't have a problem with the policy per se, especially because the teacher's are on tight time tables with the break/lunch times. I would like the scenario to copy machines. In our school, we have access to any copy machine in the building, but if a teacher comes in and needs copies, I have to stop what I'm doing and let the teacher make his/her copies, and then continue what I'm doing. That makes perfect sense, and I would think that using the microwave would fit perfectly in place of the copy machine scenario.
What is the PTO storing in the staff refrigerator? Is there a walk in type refrigerator in the cafeteria that might work better for your PTO? Was there a reason that the staff voted to keep the lounge staff only? Perhaps people were storing large amounts in the refrigerators and there was no room for the staff to effectively utilize the space that was originally purchased for them? Or were people leaving things in there and forgetting about them, making it a less than desirable place to store their lunch?
Maybe your PTO could fundraise for it's own refrigerator and that would elimate the problem entirely. Just put a sign on it that says "PTO." That's what we have.
Sorry if this was long winded or all over the place; recovering from surgery and on some pretty lovely painkillers