Presumably parent is offended by the privacy violation. It allows developer of app A to know I have app B installed, which can be used to target adds etc.
While unlikely, it could be used for worse. Let's say a local or niche publication has an app and can detect if any of its subscribers are also using Grindr, Tinder, or some sort of app that signifies, to them, some cardinal sin on the part of the subscriber. That could be used to blackmail/shame/harass people.