I read somewhere that it's an FCC regulation or something that requires manufacturers to have a shutter sound to help prevent things like using it like a peeping tom. Or something to that effect. All I know is I've yet to have a smart phone camera that had a silent shutter setting on the OEM app. I've always had to silence the phone itself. There's also third party apps in Play with no shutter sound, but I can't vouch for any personally.
This is what we were hearing a few years back. The idea of photographing someone without their knowledge was bigger then because we didn't all know that a phone facing you meant a camera facing you. While the camera thing had become really common place with our every day lives I don't think it is as big of a deal as it was then, but it might still be.
Personally I like the noise because I don't care what others think I am doing and why, I don't take pictures of strangers and I like to hear when the actual shot went off so I know what I captured.
----- completely unrelated story about cameras and other people... - - - -
I live in South Florida.. I like to go to the beach. I got a new awesome camera for my birthday which also happens to fall right around Memorial Day weekend. During that weekend the beaches are obviously packed. So is the water with boats and planes flying over it... We were standing, me my wife and 3 kids looking for somewhere to go and get some ocean pictures. We came to a stop behind 3 women wearing bikinis.. I actually wasn't focused on women, weird, but more on what awesome photos might be out in the water... I started shooting over everyone at boats and jet skis... I am shooting a load of shots and the girls are getting up to leave... I notice them moving, it I don't care and I am doing my thing. My wife tells me later that they were looking at me as if I were shooting them and watching them dress... At that point while I am looking through the camera above them, they aren't even in my camera view one of most guys bucket list things happen, one of the women... Well... Fell out if her suit... I am still looking through the camera, not with them in view and I hear my wife say wow. I am thinking something cool was out there to take a picture if so I go from my camera to turn around and ask her what... She says she'll tell me later... I go back to shooting the water... Missed the whole thing and never even knew it was happening until 20 minutes later when I was back at the car... Talk about perfect time perfect situation (totally creepy I get it) and missed everything! I totally would not have snapped a real picture if that... Probably emblazoned it into the permanent memory file, but nope... Sail boat for me!