iMessages, facebook, whatsapp, ... Unfortunately every messaging platform (except old school texts, but iPhones will automatically send iMessages to other iPhones) seem to have implemented this "feature"
It's optional in iMessages, I believe it's opt-in (not 100%, changed the setting a long time ago to have it on, personal preference).
That said, it was creepy when I realized the only reason it was enabled on my ex-girlfriend's phone (she didn't like it and is tech-illiterate) was that her previous boyfriend had enabled it to see if/when she read his messages (creepy dude that'd send her flowers randomly two years after their break up, but she thought it was sweet and that they were "just friends", yeah, sure).
I am assuming this is an iPhone "feature"? Creepy. Can it be disabled?