Is it that different than your boss overhearing you discussing your medical conditions on your work phone?
If you don't want them to know about it, don't use their equipment to communicate about it.
Airlines have had wired phones on seat backs for years without any problems or public outcry. The only difference between them and cell phones are that the users don't have to pay the airline to use their cell phones.
Sure it's annoying to have the person next to you yacking on a cell phone, but it's also annoying to have them snoring, talking loudly, crying, etc.
We need to stop making a big deal out of the fact the we're annoyed and appreciate the fact that we can travel thousands of miles without anybody die from scurvy, malnutrition, or attacks from unfriendly people or wild animals.