Of course it's on the user, as if the phone was on a pillow and not being used there is no way it would bend. The fact is, smartphones are something we carry with us 24/7. Apple knows it, hell they were the ones who really pushed the need to have smartphones. So they should know that people are going to put phones in their pocket when they are out and about, and also know of the forces that are typically present when a phone is in a pocket when sitting down, running, etc.
There is no reason it should bend so easily. If users are complaining they had no idea how it could have bent, there wasn't that much force on the phone, otherwise they would feel it against their legs. Most of us have had smartphones for years, some even Android phones with the same size or larger screens, and none of those had bent.