They belong to you, regardless of who paid for them. I can't see how it would be illegal or unethical to give them away. You haven't tried to commit insurance fraud, you just have extras. I think it would be unethical to toss them if they can help someone else. Do you know if they are still good when they are expired?
These guys don't seem to take sensors, but maybe they can give you advice: http://www.cr3diabetes.org/Donate.aspx
We had some and gave them to a friend who ran out. He said they were still good past expiration 6 mo.