I couldn't agree with you more. A few months ago when I spoke to my grandmother, I told her that I'd been diagnosed with clinical depression and anxiety attacks, and her first reaction was, "You need to tell your mother. She needs to be there for you. She deserves to know these things. Otherwise you're going to have a mental breakdown, dear."
I found out that she'd called my mother after we spoke and told her to call me, after I'd specifically told her not to tell my mother. My grandmother called me back immediately, told me she called her and told her to call me, and begged me to talk to my mom when she calls. I was very angry with my grandmother and told her no. The phone call from my mother came a few minutes later. I sent it to voicemail. All it said was, "Hi, just wondering if you guys would like to come over for dinner. Call me back!" I WAS INFURIATED! As a result of all of this, I don't tell my grandmother anything important. Our relationship consists of talking about the weather and listening to her complain about her health, and that's about it (aside from listening to her attempt to change my mind about no-contact with my mother). It hurts very deeply that I can't have an actual real relationship with my grandma because of all this.